In the PDF Viewer .js, throw Error objects instead of strings.

As per the style guide.

Change-Id: Ie8519eba699a66c224fb669b782627af276d1872
Reviewed-on: https://chromium-review.googlesource.com/c/1265507Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597316}

chrome/browser/resources/pdf/pdf_viewer.js

@@ -818,7 +818,7 @@ PDFViewer.prototype = {
    */
   setUserInitiated_: function(userInitiated) {
     if (this.isUserInitiatedEvent_ == userInitiated) {
-      throw 'Trying to set user initiated to current value.';
+      throw new Error('Trying to set user initiated to current value.');
     }
     this.isUserInitiatedEvent_ = userInitiated;
   },

chrome/browser/resources/pdf/viewport.js

@@ -367,8 +367,9 @@ Viewport.prototype = {
    */
   setZoomInternal_: function(newZoom) {
     if (!this.allowedToChangeZoom_) {
-      throw 'Called Viewport.setZoomInternal_ without calling ' +
-          'Viewport.mightZoom_.';
+      throw new Error(
+          'Called Viewport.setZoomInternal_ without calling ' +
+          'Viewport.mightZoom_.');
     }
     // Record the scroll position (relative to the top-left of the window).
     var currentScrollPos = {