Support multiple contexts in command_buffer_gles2
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#356058}

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