GitLab

Various cleanup:
- Every context type except GLContextStub calls InitializeDynamicBindings in
  MakeCurrent. Make it consistent.
- In turn, it means we don't need an explicit InitializeDynamicMockBindings, so
  remove it.
- In some tests, we were creating a context just to be able to call
  InitializeDynamicMockBindings. This is not necessary any more - dynamic
  bindings will be initalized as the first useful context is made current
- Realizing that InitializeDynamicGLBindings is always called with a real
  context, remove the special cases for mock bindings. This makes it consistent
  with other GL implementations
- In turn InitializeDynamicGLBindings always does exactly
  InitializeDynamicGLBindingsGL for valid GLImplementations, and is only called
  with a valid GLImplementation, simplify. It also means it can't fail.

BUG=None
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2129043003
Cr-Commit-Position: refs/heads/master@{#404548}