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Add a new MediaLog level so that we don't have to abuse MEDIALOG_ERROR,
which could end up in MediaError.message and should be carefully used.

Also update documentation on MediaLog levels.

Also use MEDIALOG_WARNING in audio_timestamp_validator.cc for non-fatal
issues.

Bug: 843808
Test: Manually tested and verified in about://media-internals
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;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I20350d305b3f55d1f59e6d2324cc3be7452bcbbe
Reviewed-on: https://chromium-review.googlesource.com/1063009Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559399}