Group all Chromium TryServer email in single threads.
Group all Chromium TryServer email in single threads.

Before this change, we grouped all Chromium TryServer
email by result, reason, builder, and revision.  This meant
that a new thread would show up in a user's inbox for each
reason/builder/revision combination.

After this change, we only group email by result, reason,
and revision.  All of the try job results across all of
the builders within a set of result/reason/revision will
now be grouped into a single thread in a user's inbox.

R=iannucci@chromium.org,ilevy@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16294007

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@213543 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build