[Cleanup] Follow Up of Pinch/Zoom Infrastucture & Plumbing CL

Follow up of Pinch/Zoom Infrastructure & Plumbing CL to remove methods
related to max scroll offset and making clip layer methods pure virtual.

This CL is followup on https://codereview.chromium.org/23983047.

BUG=148816

Review URL: https://codereview.chromium.org/139923006

git-svn-id: svn://svn.chromium.org/blink/trunk@166454 bbb929c8-8fbe-4397-9dbb-9b2b20218538