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This just helps out the next CL in line which adds the new types for
media log and needs to serialize some of the buffering state types.

Bug: 794255
Change-Id: Id83d79108edc0021316fc362a222eadab7d3f032
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2005818Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732663}