Eliminate hit test for content detection on GestureTap.

Currently WebViewImpl does a redundant hit test for content detection even
though we've already done event targeting before it. This patch updates it to
use the hit test result from the targeted event. This also has the positive
side effect that content detection uses the touch adjusted node.

This is part 2 of 2, depends on
https://codereview.chromium.org/603813002/

BUG=381728, 416746

Review URL: https://codereview.chromium.org/604573002

git-svn-id: svn://svn.chromium.org/blink/trunk@182997 bbb929c8-8fbe-4397-9dbb-9b2b20218538