Fix MSAA on windows
MSAA would brake on windows because skia creates a BGRA8 internal format render
buffer, which we didn't support on windows.
We were turning on BGRA8_EXT for render buffers when we had
https://www.opengl.org/registry/specs/EXT/bgra.txt, this is a desktop spec that doesn't mention BGRA8_EXT
for render buffers, but we're assuming that it should support it.
I've added the condition that BGRA8 is supported for ANGLE as well as GL_EXT_bgra, and a couple test
to confirm this.
BUG=460567
Review URL: https://codereview.chromium.org/
947653002
Cr-Commit-Position: refs/heads/master@{#317958}