Make UnacceleratedBufferSurface use a SkSurface instead of a SkCanvas

This change will be used in an other CL to resolve the attached bug.

BUG=170021

Review URL: https://codereview.chromium.org/392383005

git-svn-id: svn://svn.chromium.org/blink/trunk@178356 bbb929c8-8fbe-4397-9dbb-9b2b20218538