Update message loop start time histogram name in startup.py.
Update message loop start time histogram name in startup.py.

The Startup.BrowserMessageLoopStartTimeFromMainEntry histogram
was replaced by Startup.BrowserMessageLoopStartTimeFromMainEntry2
(crbug.com/634408).

BUG=chromium:679967

Review-Url: https://codereview.chromium.org/2690873006

sahiljain/catapult,catapult-project/catapult,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult