Added check in page click tracker to ensure events passed in are mouse events

BUG=61255
TEST=None

Review URL: http://codereview.chromium.org/4120011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64510 0039d316-1c4b-4281-b951-d872f2087c98
void PageClickTracker::StartTrackingFrame(WebFrame* frame) {
  tracked_frames_.push_back(frame);
  frame->document().addEventListener("mousedown", this, false);
}

void PageClickTracker::StartTrackingFrame(WebFrame* frame) {
  tracked_frames_.push_back(frame);
  frame->document().addEventListener("mousedown", this, false);
}
