[telemetry] Speed index - calculate histogram difference using earth mover's distance.

Previously it was the total difference of the individual buckets.
The EMD is better for noisy data, as would be caused by compression artifacts and things like that.


BUG=None.
TEST=None.

Review URL: https://codereview.chromium.org/160163002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252495 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252495