Should use (0, 0) for scroll layer position in threaded overflow scrolling

The scroll offset on compositing layers are adjusted (has non-negative value),
so the neutral position (without scroll compensation) of the layer should
be always aligned to top-left corner.

R=enne
BUG=330635

Review URL: https://codereview.chromium.org/130593002

git-svn-id: svn://svn.chromium.org/blink/trunk@164835 bbb929c8-8fbe-4397-9dbb-9b2b20218538