Send compressed histograms with system logs when sending feedback.

Send the metrics in JSON format so they can be imported and displayed
by the histograms page (in a forthcoming CL).

BUG=237288
R=rkc@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/55363003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235981 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235981