Makes the extension resize gripper only visible when the mouse is over
it.

BUG=45750
TEST=see bug

Review URL: http://codereview.chromium.org/2800022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50515 0039d316-1c4b-4281-b951-d872f2087c98
 ResizeGripper::~ResizeGripper() {
}

 ResizeGripper::~ResizeGripper() {
}
