[Telemetry] Measure power in tab_switching measurement

* Measure power instead of CPU usage in tab switching measurement.
* Fix a bug in said measurement where the initial open tab in the browser window was not navigated.
* Add a benchmark that switches between 5 blank tabs.

BUG=339180

Review URL: https://codereview.chromium.org/178363005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255925 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255925