Add support for creating sparse histograms to Platform

This allows creating sparse UMA histograms in Blink. The
chromium side of this is http://crrev.com/212717.

BUG=139612

Review URL: https://chromiumcodereview.appspot.com/19490005

git-svn-id: svn://svn.chromium.org/blink/trunk@154704 bbb929c8-8fbe-4397-9dbb-9b2b20218538