Cherry-pick: aw: Use minimum_accelerated_2d_canvas_size to turn canvas on/off

Cherry-pick of https://codereview.chromium.org/211623005/

Original description:

Always set accelerated_2d_canvas_enabled and use
minimum_accelerated_2d_canvas_size to turn canvas on and off. This is a
slight abuse but avoids accelerated_2d_canvas_enabled setting also
affecting compositing mode in blink

BUG: 13635247
Change-Id: Iba70bc922fe194c0396613023581cb3f731b5e5e
1 file changed