Fix hit-rect position for layers contained in a scrolled layer

As for http://src.chromium.org/viewvc/blink?view=rev&rev=157479, we need to
ensure co-ordinates are relative to the scrolled content, not the scrolling
container.  This handles the case when projecting layers to their enclosing
composited layer.

BUG=295337
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/24200007

git-svn-id: svn://svn.chromium.org/blink/trunk@158143 bbb929c8-8fbe-4397-9dbb-9b2b20218538