Add a command line switch for emulating shader precision
The switch sets Chrome to universally turn on ANGLE's precision
emulation. Currently it works only with the GL backends
(--use-gl=desktop on Windows). This feature is useful for testing and
debugging WebGL rendering corruption issues that appear on mobile
platforms.
Also moves gl-shader-interm-output switch to gpu_switches, where it
belongs.
BUG=438607
Review URL: https://codereview.chromium.org/
784273003
Cr-Commit-Position: refs/heads/master@{#307880}