Check for modifiers before creating comment

In some cases, the "press c to comment" feature was overriding default
browser copy behavior. This change checks for modifiers before creating
comments. This functionality is factored out into a function for future
use.

Bug: Issue 3989
Change-Id: I3ebe0dec2f5436b8339d81d99d1287799ff3568b

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
index 14fd2b7..1eb3f95 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
@@ -509,5 +509,13 @@
       assert.equal(element.$.cursor.initialLineNumber, 345);
       assert.equal(element.$.cursor.side, 'left');
     });
+
+    test('_checkForModifiers', function() {
+      assert.isTrue(element._checkForModifiers({altKey: true}));
+      assert.isTrue(element._checkForModifiers({ctrlKey: true}));
+      assert.isTrue(element._checkForModifiers({metaKey: true}));
+      assert.isTrue(element._checkForModifiers({shiftKey: true}));
+      assert.isFalse(element._checkForModifiers({}));
+    });
   });
 </script>