Use entry 0 of the GPU.BlacklistTestResultsPerEntry to keep total count.
Every time we record entries in the GPU.BlacklistTestResultsPerEntry histogram,
increment the zero'th entry. This will provide a total count for the number of
times we've recorded data in the histogram and a convenient denominator to compute
blacklist percentages per entry.
BUG=258654
Review URL: https://chromiumcodereview.appspot.com/
18501016
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@211051 0039d316-1c4b-4281-b951-d872f2087c98