Support multiple contexts in command_buffer_gles2

Fix problems with multiple contexts when using command_buffer_gles2
library.

Rendering to two different command_buffer_gles2-provided contexts would
not work, as the last created native context would remain current.

Also fixes a problem with Skia, where Skia would create and use a GLX
context directly, as well as using command_buffer_gles2 context.

BUG=skia:2992

Review URL: https://codereview.chromium.org/1416043005

Cr-Commit-Position: refs/heads/master@{#356058}
1 file changed