Remove OverlapTestRequestClient

The only implementor of OverlapTestRequestClient is RenderWidget. We should just
remove this abstract interface and use RenderWidget directly. This makes the code
simpler and removes the virtual calls.

Review URL: https://chromiumcodereview.appspot.com/22310002

git-svn-id: svn://svn.chromium.org/blink/trunk@155561 bbb929c8-8fbe-4397-9dbb-9b2b20218538