[Mac] Log SuggestedTextTouchBarController Text Suggestion Selections

A touch bar action, TEXT_SUGGESTION, was added to the TouchBar UMA
histogram to record the number of times a user selects a text
suggestion from the touch bar.

When a user selects a text suggestion from the touch bar, the
SuggestedTextTouchBarController will now log the selection using the
new TEXT_SUGGESTION histogram category.

A unit test was added to test that the selection of a text suggestion
is logged as expected.

Change-Id: I156f6179214a7f9213b22b68b4edd3f1292b77db
Reviewed-on: https://chromium-review.googlesource.com/1141212
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sarah Chan <spqchan@chromium.org>
Commit-Queue: Tessa Nijssen <tnijssen@google.com>
Cr-Commit-Position: refs/heads/master@{#576576}
3 files changed