Report difference in browser memory usage from Start() to Stop() of the metric.
Similar to how histogram memory usage is reported as difference between Start() and Stop() of the metric. It makes sense for some tests to measure only the change in memory usage for a test page instead of just the final browser memory.
Since, some tests might still want to report only the end browser memory usage, a "report_memory_delta" argument is added to memory metric.
BUG=324951
Review URL: https://codereview.chromium.org/
106023003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260648 0039d316-1c4b-4281-b951-d872f2087c98