Update histograms.xml to include the latest policies.

The policy documentation has been updated to include histograms.xml as one of
the files the needs to be updated; this change add a couple of policies that
weren't added to the file.

Review URL: https://codereview.chromium.org/43733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231329 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231329