Implement bump-scroll browser-test.

remoting/webapp/js_proto/dom_proto.js

Index: remoting/webapp/js_proto/dom_proto.js
diff --git a/remoting/webapp/js_proto/dom_proto.js b/remoting/webapp/js_proto/dom_proto.js
index 8538957574adaa140fd56c5316c7865e4bdfc6d3..d7f475d244359d157071e95b7087dcc105d4b671 100644
--- a/remoting/webapp/js_proto/dom_proto.js
+++ b/remoting/webapp/js_proto/dom_proto.js
@@ -219,3 +219,26 @@ Promise.reject = function (reason) {};
  * @return {Promise}
  */
 Promise.resolve = function (value) {};
+
+/**
+ * @param {string} type
+ * @param {boolean} canBubble
+ * @param {boolean} cancelable
+ * @param {Window} view
+ * @param {number} detail
+ * @param {number} screenX
+ * @param {number} screenY
+ * @param {number} clientX
+ * @param {number} clientY
+ * @param {boolean} ctrlKey
+ * @param {boolean} altKey
+ * @param {boolean} shiftKey
+ * @param {boolean} metaKey
+ * @param {number} button
+ * @param {EventTarget} relatedTarget
+ */
+Event.prototype.initMouseEvent = function(
+    type, canBubble, cancelable, view, detail,
+    screenX, screenY, clientX, clientY,
+    ctrlKey, altKey, shiftKey, metaKey,
+    button, relatedTarget) {};