(Merging on behalf of mdw@)

(Merging on behalf of mdw@)

Add UMA to count the number of Google CAPTCHA pages shown and solved by users.

This is intended to track down problems where a large number of mobile
users in certain countries are apparently seeing CAPTCHA pages.
(http://b2.corp.google.com/23081866 for context.)

I realize this may not be the ideal place for this code - happy for suggestions as to where else this might go.

BUG=552470
Review URL: https://codereview.chromium.org/1441393002

Cr-Commit-Position: refs/heads/master@{#360011}
(cherry picked from commit 71c7cacbdb9cd4fa161909a72a9157f574a2b775)

Committed: https://chromium.googlesource.com/chromium/src/+/773e3aadb0817084f074e190662205304064f9ae

[maniscalco, 2015-11-19 17:28:08]

Committed patchset #1 (id:1) manually as
773e3aadb0817084f074e190662205304064f9ae.