Return width from drawText to stop re-measure in Canvas

Allowing drawText to return width of what was drawn, 
so that we don't have to make call to width to 
re-measure in Canvas drawing.

R=eae@chromium.org
BUG=410706
TEST=TEST=fast/canvas/canvas-drawtext-width-return.html

Review URL: https://codereview.chromium.org/519373002

git-svn-id: svn://svn.chromium.org/blink/trunk@181703 bbb929c8-8fbe-4397-9dbb-9b2b20218538