Match Pepper UMA histograms by prefix

Previously each individual histogram name had to be hashed and whitelisted.
This had the drawback of older versions of Chrome not knowing about newer
histogram names which were added, and then would be discarded.  Now we match
against known prefixes to allow a broader range of future histogram names.

BUG=317833
R=bbudge@chromium.org, isherman@chromium.org

Review URL: https://codereview.chromium.org/187593009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255487
Add missing StatisticsProvider::SetTestProvider(NULL)

BUG=349948
TEST=unit_tests,browser_tests
R=asargent@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/177923010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255488 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255488