Set up viewport and scissor according to the actual drawing buffer size.

BUG=303915
TEST=drawingbuffer-test.html on mac amd or intel
R=bajones@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/164843005

git-svn-id: svn://svn.chromium.org/blink/trunk@167432 bbb929c8-8fbe-4397-9dbb-9b2b20218538