Enabling test case to write the average+stddev pair to the results

The sketch test case writes avg+stddev for all its metrics:
	<key>fetch-frames</key>
	<dict>
		<key>description</key>
		<string>time to dump backtrace for every frame in every thread</string>
		<key>stddev</key>
		<real>0.006270938361432314</real>
		<key>value</key>
		<real>0.011568079851851851</real>
	</dict>

llvm-svn: 179550