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This is done by updating the histograms config model and by uncommenting
the call to ExpandHistogramsOWNERS() in merge_xml.py. This ensures
<owner>src/dir/OWNERS</owner> is expanded into actual owners, e.g.
<owner>name1@google.com</owner> and <owner>name2@chromium.org</owner>.
Uncommenting this allows components to be extracted from OWNERS files as
well. That way, component info, if available in an OWNERS file, is added
to histogram expiry bugs, e.g. http://crbug/1088985.

Also, add component documentation and add a component to two
Variations.Headers.* histograms.

Support was previously added for component tags, see http://crbug/989112
for details, but it wasn't explicitly used.

Bug: 1133699
Change-Id: I572c877be8929923fd14df352e762a7a293ca5ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442136Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#813914}