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Work is being done on shelf notification indicators as a separate
feature from context menu notifications, so create a new flag to
separate the two features.

Bug: 1080827
Change-Id: I9e6a2a20a30b692a5e9c185b904b0ff337eaad39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2228189
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774726}