[Telemetry] Add power measurement to more benchmarks

Add power measurement to the following benchmarks:
- dromaeo
- image_decoding
- indexeddb_perf
- jsgamebench
- kraken
- media
- memory
- octane
- page_cycler
- robohornet_pro
- spaceport
- sunspider

BUG=336672

Review URL: https://codereview.chromium.org/145273016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247384 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247384
Re-use the result of ValueAsString where possible in PrefHashCalculator.

Whenever the initial hash doesn't match; ValueAsString was re-computed to check the legacy hash.

Since this is on the startup path and ValueAsString is the most expensive call in these checks, avoid computing it twice where possible.

BUG=330908

Review URL: https://codereview.chromium.org/148493003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247385 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247385