Add comments with bug reference to previously disabled tests.

Tests were disabled in https://chromium-review.googlesource.com/707011.

TBR=dtseng@chromium.org

Bug: 751046
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I1289bc557ad0a3409e174083c861a7e5151d0006
Reviewed-on: https://chromium-review.googlesource.com/707095
Reviewed-by: Henrik Grunell <grunell@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507368}

chrome/browser/resources/chromeos/chromevox/cvox2/background/panel_test.extjs

@@ -65,6 +65,7 @@ PanelTest.prototype = {
   */}
 };
 
+// Flaky. http://crbug.com/751046.
 TEST_F('PanelTest', 'DISABLED_ActivateMenu', function() {
   var mockFeedback = this.createMockFeedback();
   this.runWithLoadedTree(this.linksDoc, function(root) {
@@ -81,6 +82,7 @@ TEST_F('PanelTest', 'DISABLED_ActivateMenu', function() {
   });
 });
 
+// Flaky. http://crbug.com/751046.
 TEST_F('PanelTest', 'DISABLED_LinkMenu', function() {
   var mockFeedback = this.createMockFeedback();
   this.runWithLoadedTree(this.linksDoc, function(root) {