gpu: Added the feature flag for GL_EXT_texture_rg to FeatureInfo
* GL_EXT_texture_rg & GL_ARB_texture_rg both add the extension GL_EXT_texture_rg
* validators for GL_RED_EXT and GL_RG_EXT added for textures and read pixels
* validators for GL_R8_EXT and GL_RG8_EXT added for render buffer
* format validators added for unsigned byte, float (if supported) and half float
(if supported) added
* unit tests for EXT and ARB + validators
* unit test to confirm that the extension is available on actual hardware
BUG=435379
Review URL: https://codereview.chromium.org/
751113002
Cr-Commit-Position: refs/heads/master@{#306337}