Improving GestureNav screenshotting performance - Part 1 - gl_helper

Improving GestureNav screenshotting performance - Part 1 - gl_helper

Pixel readback in GL_RGBA format can be a very expensive operation on
some hardware configurations.

Adding support for kAlpha_8_SkColorType to CropScaleReadbackAndCleanTexture().
For this color type, the texture is converted to grayscale on the GPU and then
read back using GL_BGRA_EXT format.
This improves the readback performance because we read four times less
data and we don't have to do any format conversions as we are reading.

Original review: https://codereview.chromium.org/384453002/
Part 2: https://codereview.chromium.org/389683002/

BUG=379983, 380779

Review URL: https://codereview.chromium.org/388953002

Cr-Commit-Position: refs/heads/master@{#291768}