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This is the last class in the flags package in chrome_java.

I considered moving it to a new metrics module, but the enums like
ActivityType are core and used in a lot of places, and have meaning
besides metrics, so I'm keeping these in flags and simply
modularizing to unblock any code that uses them.

Change-Id: I57c6accfaa2f1a18e230af2219b5e883919af49b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625890Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843695}