Added a glBench for testing performance of vec4 vs scalar coverage in generated shaders.

Added bench for timing vec4 vs scalar type for coverage in shaders

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/6104ced165f17eb2f765ace354d5895c0bc890c5

Review URL: https://codereview.chromium.org/1225383002
1 file changed