Optimize text layer: Avoid unneeded .textContent

Built-in DOM properties are slower than plain JS properties.
A few lines before, textContent is assigned as follows:
  textDiv.textContent = geom.str;
So replacing textDiv.textContent.length with geom.str.length slightly
improves performance without side effects.

web/text_layer_builder.js

@@ -197,7 +197,7 @@ var TextLayerBuilder = (function TextLayerBuilderClosure() {
       // We don't bother scaling single-char text divs, because it has very
       // little effect on text highlighting. This makes scrolling on docs with
       // lots of such divs a lot faster.
-      if (textDiv.textContent.length > 1) {
+      if (geom.str.length > 1) {
         if (style.vertical) {
           textDiv.dataset.canvasWidth = geom.height * this.viewport.scale;
         } else {