Add ANGLE Platform implementation.

Add ANGLE Platform implementation.

This logic allows us to record traces and histograms easily within
ANGLE's code. It also gives a flexible design that we can extend
with further methods later, without breaking compilation. The
design is based on blink::Platform.

Also add two new histogram descriptions to histograms.xml, and
move the GPU Process metrics init to before InitializeOneOff, so
ANGLE can record histograms during Renderer initialization.

BUG=436191

Review URL: https://codereview.chromium.org/923223002

Cr-Commit-Position: refs/heads/master@{#319930}