[telemetry] Add new measurement that counts number of GCs needed to free V8 context.

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."
   },
   "bit/s": {
     "improvement_direction": "down",
     "why": "Bitrate."
@@ skipping 37 matching lines @@
     "improvement_direction": "up",
     "why": "Synonym for fps."
   },
   "frame_count": {
     "improvement_direction": "up",
     "why": "layer_tree_host_perftest"
   },
   "frame_time": {
     "improvement_direction": "down"
   },
+  "garbage_collections": {
+    "improvement_direction": "down",
+    "why": "Number of GCs needed to collect an object. Less is better."
+  },
   "Hz": {
     "improvement_direction": "up",
     "why": "Higher frequencies are faster."
   },
   "janks": {
     "improvement_direction": "down",
     "why": "Fewer janks is better."
   },
   "kb": {
     "improvement_direction": "down",
@@ skipping 123 matching lines @@
   },
   "vsyncs": {
     "improvement_direction": "down",
     "why": "Used in smoothness benchmarks. Number of vsyncs to generate a frame, never < 1.0"
   },
   "idle%": {
     "improvement_direction": "up",
     "why": "Percentage of work done in idle time."
   }
 }