Move GLSurfaceOSMesaX11 into it's own source file.

The initialization code in GLSurfaceOSMesaX11 needs to be used from
//ui/gl/init and as such can't be in anonymous namespace. Expose the
class similar to other GLSurface implementations.

BUG=611142

Review-Url: https://codereview.chromium.org/2022103002
Cr-Commit-Position: refs/heads/master@{#396911}
5 files changed