[api-minor] Add an option to PDFJS for specifying the |target| attribute of external links

Replaces `PDFJS.openExternalLinksInNewWindow` with a more generic configuration option.
*Note:* `PDFJS.openExternalLinksInNewWindow = true;` is equal to `PDFJS.externalLinkTarget = PDFJS.LinkTarget.BLANK;`.

src/display/annotation_helper.js

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 /* globals PDFJS, Util, AnnotationType, AnnotationBorderStyleType, warn,
-           CustomStyle */
+           CustomStyle, isExternalLinkTargetSet, LinkTargetStringMap */
 
 'use strict';
 
@@ -261,8 +261,9 @@ var AnnotationUtils = (function AnnotationUtilsClosure() {
 
     var link = document.createElement('a');
     link.href = link.title = item.url || '';
-    if (item.url && PDFJS.openExternalLinksInNewWindow) {
-      link.target = '_blank';
+
+    if (item.url && isExternalLinkTargetSet()) {
+      link.target = LinkTargetStringMap[PDFJS.externalLinkTarget];
     }
 
     container.appendChild(link);

src/display/api.js

@@ -169,14 +169,27 @@ PDFJS.maxCanvasPixels = (PDFJS.maxCanvasPixels === undefined ?
                          16777216 : PDFJS.maxCanvasPixels);
 
 /**
- * Opens external links in a new window if enabled. The default behavior opens
- * external links in the PDF.js window.
+ * (Deprecated) Opens external links in a new window if enabled.
+ * The default behavior opens external links in the PDF.js window.
  * @var {boolean}
  */
 PDFJS.openExternalLinksInNewWindow = (
   PDFJS.openExternalLinksInNewWindow === undefined ?
     false : PDFJS.openExternalLinksInNewWindow);
 
+/**
+ * Specifies the |target| attribute for external links.
+ * The constants from PDFJS.LinkTarget should be used:
+ *  - NONE [default]
+ *  - SELF
+ *  - BLANK
+ *  - PARENT
+ *  - TOP
+ * @var {number}
+ */
+PDFJS.externalLinkTarget = (PDFJS.externalLinkTarget === undefined ?
+                            PDFJS.LinkTarget.NONE : PDFJS.externalLinkTarget);
+
 /**
   * Determines if we can eval strings as JS. Primarily used to improve
   * performance for font rendering.

src/shared/util.js

@@ -328,6 +328,49 @@ function shadow(obj, prop, value) {
 }
 PDFJS.shadow = shadow;
 
+var LinkTarget = PDFJS.LinkTarget = {
+  NONE: 0, // Default value.
+  SELF: 1,
+  BLANK: 2,
+  PARENT: 3,
+  TOP: 4,
+};
+var LinkTargetStringMap = [
+  '',
+  '_self',
+  '_blank',
+  '_parent',
+  '_top'
+];
+
+function isExternalLinkTargetSet() {
+//#if GENERIC
+  if (PDFJS.openExternalLinksInNewWindow) {
+    warn('PDFJS.openExternalLinksInNewWindow is deprecated, ' +
+         'use PDFJS.externalLinkTarget instead.');
+    if (PDFJS.externalLinkTarget === LinkTarget.NONE) {
+      PDFJS.externalLinkTarget = LinkTarget.BLANK;
+    }
+    // Reset the deprecated parameter, to suppress further warnings.
+    PDFJS.openExternalLinksInNewWindow = false;
+  }
+//#endif
+  switch (PDFJS.externalLinkTarget) {
+    case LinkTarget.NONE:
+      return false;
+    case LinkTarget.SELF:
+    case LinkTarget.BLANK:
+    case LinkTarget.PARENT:
+    case LinkTarget.TOP:
+      return true;
+  }
+  warn('PDFJS.externalLinkTarget is invalid: ' + PDFJS.externalLinkTarget);
+  // Reset the external link target, to suppress further warnings.
+  PDFJS.externalLinkTarget = LinkTarget.NONE;
+  return false;
+}
+PDFJS.isExternalLinkTargetSet = isExternalLinkTargetSet;
+
 var PasswordResponses = PDFJS.PasswordResponses = {
   NEED_PASSWORD: 1,
   INCORRECT_PASSWORD: 2