Fix dm with Chromium command buffer driver

Fix a crash in dm when run with Chromium command buffer driver.

Also removes glCopyTextureCHROMIUM, it does not seem to be used.
This is removed on the grounds that it would need similar change,
if it was used. The variable fCopyTextureCHROMIUM does not seem to
be populated in the Chromium side, either.

BUG=skia:2992

Review URL: https://codereview.chromium.org/1389213004
3 files changed