Reduce hit test on ShowPress by moving hit testing to WebViewImpl.

Previously there was a hit test for link highlighting in WebViewImpl and then a                                          
separate one in EventHandler::handleGestureEvent. This CL makes it so that a                                             
single hit test in WebViewImpl is reused by the EventHandler. Scroll events                                              
still need to hit test in a separate fashion though and their handling has been                                          
refactored into a separate code path.

BUG=381728

Review URL: https://codereview.chromium.org/490783003

git-svn-id: svn://svn.chromium.org/blink/trunk@182281 bbb929c8-8fbe-4397-9dbb-9b2b20218538