Prevent WebView from crashing when detached from the window

Bug #6365056

WebView enqueues a functor in the hardware renderer to handle
animations and this functor is called at a later time by the
hardware renderer. However, the functor was not removed from
the queue when WebView was removed from the window. This could
cause the hardware renderer to attempt to execute an invalid
functor and lead to a crash.

Change-Id: I9d38e80f3fdc5e29d4d0cdfa1e893c251a954508