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This makes GpuProcessHost::LaunchGpuProcess significantly easier to read.
This is in preparation for getting rid of the GPU broker process.
Also make "GPU process" capitalization consistent.

BUG=329375
TEST=content_shell boots, --disable-gpu-sandbox gets propagated.

Review URL: https://codereview.chromium.org/498833002

Cr-Commit-Position: refs/heads/master@{#291586}