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Per MTBR review, this list is growing too big to keep living inside
MediaLog; so create a new home for all watch time related keys in
a new "watch_time_keys.{h,cc} file.

BUG=none
TEST=compiles

Change-Id: Ib927f21f04d0a321dac0eec8cc81af74302334e3
Reviewed-on: https://chromium-review.googlesource.com/521804Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476489}