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Move GL one-off initialization from //ui/gl to //ui/gl/init. Create a
new class GLInitializer to contain the platform specific initialization
methods and add platform specific implementation in a conditionally
included file per platform. Remove the existing one-off initialization
code in GLSurface and from platform specific GLSurface files.

This change is part of step two from the associated bug.

BUG=611142

Review-Url: https://codereview.chromium.org/2024953002
Cr-Commit-Position: refs/heads/master@{#397839}