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If the provider is not null, it will have a context that is not null.
The provider is a WebGraphicsContext3DProviderImpl, which is given
a ContextProviderWebContext.

The ContextProviderWebContext is a ContextProviderCommandBuffer, the
factory for which returns a null provider if the underlying context
is null.

Further, the provider comes from RenderThreadImpl's
SharedMainThreadContextProvider(), which will return null if
the provider is null, or if the provider fails to initialize its
GL context.

R=junov@chromium.org, kbr@chromium.org
BUG=584497

Review URL: https://codereview.chromium.org/1813603003

Cr-Commit-Position: refs/heads/master@{#381765}