[chromeos/about:power] Collect and display cpuidle and cpufreq stats
  
Salient changes introduced by this CL:
1. PowerDataCollector collects CPU idle and CPU freq state
   information.
2. The about:power page is organized into expandable 
   sections.
3. Two sections, "Idle State Data" and "Frequency State 
   Data" are added to the page.
4. Plots have a legend.

BUG=335816

Review URL: https://codereview.chromium.org/149973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256294 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256294