Simplify the updatetextlayermatches event handling in TextLayerBuilder

This implements the nice suggestion from https://github.com/mozilla/pdf.js/pull/10099#discussion_r219698000, which I at the time didn't think would work.
I'll probably have to plead temporary insanity here, since it *should* have been totally obvious to me how this could be implemented. By simply not registering the event until the `textLayer` is actually rendered, removing the event on `cancel` works just fine.

This patch also removes the `pagecancelled` event, given that it's no longer used anywhere in the code-base and that its implemention was flawed since it wasn't guaranteed to be called in *every* situation where rendering was actually cancelled.

web/pdf_page_view.js

@@ -259,8 +259,6 @@ class PDFPageView {
   }
 
   cancelRendering(keepAnnotations = false) {
-    const renderingState = this.renderingState;
-
     if (this.paintTask) {
       this.paintTask.cancel();
       this.paintTask = null;
@@ -276,14 +274,6 @@ class PDFPageView {
       this.annotationLayer.cancel();
       this.annotationLayer = null;
     }
-
-    if (renderingState !== RenderingStates.INITIAL) {
-      this.eventBus.dispatch('pagecancelled', {
-        source: this,
-        pageNumber: this.id,
-        renderingState,
-      });
-    }
   }
 
   cssTransform(target, redrawAnnotations = false) {

web/text_layer_builder.js

@@ -53,9 +53,7 @@ class TextLayerBuilder {
     this.textLayerRenderTask = null;
     this.enhanceTextSelection = enhanceTextSelection;
 
-    this._boundEvents = Object.create(null);
+    this._onUpdateTextLayerMatches = null;
-    this._bindEvents();
-
     this._bindMouse();
   }
 
@@ -109,6 +107,16 @@ class TextLayerBuilder {
     }, function (reason) {
       // Cancelled or failed to render text layer; skipping errors.
     });
+
+    if (!this._onUpdateTextLayerMatches) {
+      this._onUpdateTextLayerMatches = (evt) => {
+        if (evt.pageIndex === this.pageIdx || evt.pageIndex === -1) {
+          this._updateMatches();
+        }
+      };
+      this.eventBus.on('updatetextlayermatches',
+                       this._onUpdateTextLayerMatches);
+    }
   }
 
   /**
@@ -119,6 +127,11 @@ class TextLayerBuilder {
       this.textLayerRenderTask.cancel();
       this.textLayerRenderTask = null;
     }
+    if (this._onUpdateTextLayerMatches) {
+      this.eventBus.off('updatetextlayermatches',
+                        this._onUpdateTextLayerMatches);
+      this._onUpdateTextLayerMatches = null;
+    }
   }
 
   setTextContentStream(readableStream) {
@@ -314,40 +327,6 @@ class TextLayerBuilder {
     this._renderMatches(this.matches);
   }
 
-  /**
-   * @private
-   */
-  _bindEvents() {
-    const { eventBus, _boundEvents, } = this;
-
-    _boundEvents.pageCancelled = (evt) => {
-      if (evt.pageNumber !== this.pageNumber) {
-        return;
-      }
-      if (this.textLayerRenderTask) {
-        console.error('TextLayerBuilder._bindEvents: `this.cancel()` should ' +
-          'have been called when the page was reset, or rendering cancelled.');
-        return;
-      }
-      // Ensure that all event listeners are cleaned up when the page is reset,
-      // since re-rendering will create new `TextLayerBuilder` instances and the
-      // number of (stale) event listeners would otherwise grow without bound.
-      for (const name in _boundEvents) {
-        eventBus.off(name.toLowerCase(), _boundEvents[name]);
-        delete _boundEvents[name];
-      }
-    };
-    _boundEvents.updateTextLayerMatches = (evt) => {
-      if (evt.pageIndex !== this.pageIdx && evt.pageIndex !== -1) {
-        return;
-      }
-      this._updateMatches();
-    };
-
-    eventBus.on('pagecancelled', _boundEvents.pageCancelled);
-    eventBus.on('updatetextlayermatches', _boundEvents.updateTextLayerMatches);
-  }
-
   /**
    * Improves text selection by adding an additional div where the mouse was
    * clicked. This reduces flickering of the content if the mouse is slowly