The registered histograms on the native side are exposed
as JSON text from the FeedbackCollector. The histograms are
exposed only if:
(i)  "Include screenshot and logs" is checked when
the feedback was collected, and
(ii) The feedback is submitted from a non-incognito
profile.

In the next CL, these would be attached to the feedback
report.

BUG=651621

Review-Url: https://codereview.chromium.org/2378773010
Cr-Commit-Position: refs/heads/master@{#422765}