Rename PerformanceMeasure argument duration->Endtime

This is already being passed endTime instead of duration, the argument is just named incorrectly.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22907007

git-svn-id: svn://svn.chromium.org/blink/trunk@156184 bbb929c8-8fbe-4397-9dbb-9b2b20218538