Use GLHelper for glReadPixels and format detection
GLHelper has functionality for testing for appropriate GL readback
format support, caching this information where appropriate. This change
switches RendererGpuVideoAcceleratorFactories over to using that code
for supported format identification and glReadPixels in the desired
format. The code continues to do a software swizzle if necessary.
TEST=WebGL conformance tests
BUG=None
Review URL: https://codereview.chromium.org/
456823002
Cr-Commit-Position: refs/heads/master@{#293358}