Remove WebLayer::maxScrollPosition().

This code is only used in a test, and cc-side support is going away, so
remove it in Blink.

Review URL: https://codereview.chromium.org/215033003

git-svn-id: svn://svn.chromium.org/blink/trunk@170480 bbb929c8-8fbe-4397-9dbb-9b2b20218538