Return a NULL histogram pointer on construction error

Check the pointer returned for the metricsPrivate API to keep from crashing Chrome with bad data.

BUG=334135

Review URL: https://codereview.chromium.org/141393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256822 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256822