Remove setting for disabling cc touch hit testing.

Compositor touch hit testing was re-enabled on Android webview back in February
(https://codereview.chromium.org/183943002) and we haven't had any complaints
since. It's probably safe to remove the flag and related code now.

This is part 2 of 2, depends on Chromium CL:
https://codereview.chromium.org/399953004

BUG=304518

Review URL: https://codereview.chromium.org/406053002

git-svn-id: svn://svn.chromium.org/blink/trunk@178806 bbb929c8-8fbe-4397-9dbb-9b2b20218538