Scissor rect on drawinfo

BUG=skia:

Review URL: https://codereview.chromium.org/678683005
Reenable assertions that were reported failing in tests/CanvasTest.cpp

BUG=skia:498,skia:499

Review URL: https://codereview.chromium.org/670403002
Working patch to remove constant attributes.  This may cause some gm mismatches, I will rebaseline tonight.

BUG=skia:

Review URL: https://codereview.chromium.org/678073005