Remove the extra ampersand from xhr post data string
Remove the extra ampersand from xhr post data string

This was from removing xhr token param in CL: 1369933002.

Review URL: https://codereview.chromium.org/1372153004

benschmaus/catapult,catapult-project/catapult-csm,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,SummerLW/Perf-Insight-Report,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,0x90sled/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,benschmaus/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,0x90sled/catapult,sahiljain/catapult,catapult-project/catapult,0x90sled/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report