Check for a non-null GrContext on the media::Context3D during paint.
Check for a non-null GrContext on the media::Context3D during paint.

The code checks that a ContextProvider was able to be found by checking
the |gl| field. But if the ContextProvider was created but lost, and
GrContext was not created yet, then it will fail to initialize and be
null. We should avoid trying to use this context then (and not crash).

TBR=dalecurtis@chromium.org
BUG=612087

Review-Url: https://codereview.chromium.org/2003833002
Cr-Commit-Position: refs/heads/master@{#395222}
(cherry picked from commit 53f7ec90b80cf9c2371c1570d0b1ce2e63332018)

Review URL: https://codereview.chromium.org/2025353002 .

Cr-Commit-Position: 03d981dee550804c6934fd1a1d9cfa3c172affd3@{#166}
Cr-Branched-From: e9621a186760eb36d74098d9e33e4dd91c427b34@{#394939}

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