Bug 932454. Make Atomics.h compilable with clang on Windows. r=nfroyd,jwalden

MSVC is sloppy with typedefs leaking. Clang is more strict.

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extra : rebase_source : e8000f43391f8a32fbde96d54ff11e09e49345f9
Bug 936246. Enable WEBGL_draw_buffers by default. r=jgilbert

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extra : rebase_source : 29ee857517ab6f30ef67dc57b5c2b875e136e386