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With new touch hit testing in compositor, which looks through all
the layers hitting the point, we don't need touch regions on overlay
layer anymore.

Note, that we still need main-thread scrolling to stay in sync with
page contents.

BUG=304271

Review URL: https://codereview.chromium.org/305383002

git-svn-id: svn://svn.chromium.org/blink/trunk@175578 bbb929c8-8fbe-4397-9dbb-9b2b20218538