Change endure benchmark to count by number of repetitions instead of time.

It's expected that the results should be slightly more steady. (I didn't measure this on my local machine before-hand, but after the new version of the Endure benchmarks are being run, we could see the results.)

Note: I should wait until crrev.com/187173002 is submitted and merge in those changes before submitting this.

BUG=

Review URL: https://codereview.chromium.org/183663029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256447 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256447