Fix progress bar regression on Mac

The clip rect in r183006 wasn't manually tested, and was wrong.
Specify the un-translated clip rect, which empirically produces
correct results in a variety of test scenarios (zoomed, retina,
various page locations).

Use a LocalCurrentGraphicsContext and delete ContextContainer,
because it is largely redundant.

BUG=419597

Review URL: https://codereview.chromium.org/620673006

git-svn-id: svn://svn.chromium.org/blink/trunk@183162 bbb929c8-8fbe-4397-9dbb-9b2b20218538