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Adding egregious glFlush calls fixed this issue when GPU Raster was
going through the command buffer (in issue 863817). Adding a flush
here to see if it will fix the issue.

If this does not fix the issue, then the next steps are to
* Flush in DoRasterCHROMIUM (far more frequent) to see if that fixes
  the issue.
* Add crash instrumentation to see which commands are being issued when
  we do crash.

Bug: 906453
Change-Id: I7d88ea2d74b52751f641faaf8b848e9a1358a770
Reviewed-on: https://chromium-review.googlesource.com/c/1352508
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612392}