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There's been a bunch of crashes where it appears no video frame was returned
even though status = success. This change adds additional logging and converts
some of the DCHECKs to CHECK so that the problem is caught earlier.

BUG=828148
TEST=encrypted media browser_tests pass

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ib15465179f4931860b6b6fb4192c320e09a30d7e
Reviewed-on: https://chromium-review.googlesource.com/993711Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548665}