webgl: Handle null context provider when trying to restore context

If the gpu process is gone, the context provider may be null, so
we must handle that scenario in the maybeRestoreContext method.

BUG=627240
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel
NOTRY=true

Review-Url: https://codereview.chromium.org/2142223002
Cr-Commit-Position: refs/heads/master@{#404857}
1 file changed