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Utilize the new gl_helper functionality to capture GestureNav screenshots
in grayscale, which drastically improves screencapture performance,
especially on hardware configurations with expensive RGBA readback.

Part 1: https://codereview.chromium.org/388953002/
Original review: https://codereview.chromium.org/384453002/

BUG=379983, 380779

Review URL: https://codereview.chromium.org/389683002

Cr-Commit-Position: refs/heads/master@{#291992}