In addition to LOADING_EXTENSION, WILL_HIDE, SHOWING, HIDING.

This sounds great to me.

Sounds great! Thanks for doing this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3f1e860db69b639a3eff71398ed86c6a919b710a

commit 3f1e860db69b639a3eff71398ed86c6a919b710a
Author: oka@chromium.org <oka@chromium.org>
Date: Thu Jul 20 02:31:24 2017

Export erroneously lingering intermediate keyboard states

Export intermediate keyboard states as UMA if it's
lingering too long (5 seconds).

- Made the UMA be exported on HIDDEN
- Checked chrome:histograms and confirmed only HIDDEN is exported.

Bug:  742724 
Test: manually tested with a little tweak in the code.
Change-Id: I5954f1c169441ef80b70c08b3515cee1b11bfcb2
Reviewed-on: https://chromium-review.googlesource.com/572606
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Commit-Queue: Keigo Oka <oka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488103}
[modify] https://crrev.com/3f1e860db69b639a3eff71398ed86c6a919b710a/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/3f1e860db69b639a3eff71398ed86c6a919b710a/tools/metrics/histograms/histograms.xml
[modify] https://crrev.com/3f1e860db69b639a3eff71398ed86c6a919b710a/ui/keyboard/keyboard_controller.cc
[modify] https://crrev.com/3f1e860db69b639a3eff71398ed86c6a919b710a/ui/keyboard/keyboard_controller.h