Ensure touch hit rects are cleared on layers that have no remaining rects

With this change the test results are now back to the same as what they were when using the stored changed rects instead of reading the real rects back from the compositor (http://crbug.com/270922).

BUG=271747

Review URL: https://chromiumcodereview.appspot.com/22884005

git-svn-id: svn://svn.chromium.org/blink/trunk@156451 bbb929c8-8fbe-4397-9dbb-9b2b20218538