[Telemetry] Add power usage metric

tools/perf/unit-info.json

 {
   "_description" : "This file contains info about our performance test units, used by the perf dashboard (http://chromeperf.appspot.com).",
 
   "%": {
     "improvement_direction": "down",
     "why": "Percent CPU usage. Used by cpu metric."
   },
   "bytes": {
     "improvement_direction": "down"
   },
@@ skipping 50 matching lines @@
     "improvement_direction": "down",
     "why": "Used in many Telemetry measurements. Fewer ms of time means faster."
   },
   "milliseconds": {
     "improvement_direction": "down",
     "why": "Synonym for ms."
   },
   "milliseconds-per-frame": {
     "improvement_direction": "down"
   },
+  "mWh": {
+    "improvement_direction": "down",
+    "why": "Fewer mWh means less power."

[tonyg, 2014/01/23 19:11:24]

Let's say: "Fewer milliwatt-hours means less energy consumed."

+  },
   "objects (bigger is better)": {
     "improvement_direction": "up",
     "why": "Used in spaceport benchmark."
   },
   "ObjectsAt30FPS": {
     "improvement_direction": "up"
   },
   "percent": {
     "improvement_direction": "down",
     "why": "Synonym for %, used in memory metric for percent fragmentation."
@@ skipping 43 matching lines @@
   "sec": {
     "improvement_direction": "down"
   },
   "seconds": {
     "improvement_direction": "down"
   },
   "us": {
     "improvement_direction": "down"
   }
 }