Update unit info file for perf dashboard.

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"
+    "why": "Percent CPU usage. Used by cpu metric."
   },
   "bytes": {
     "improvement_direction": "down"
   },
   "commit_count": {
     "improvement_direction": "up",
     "why": "layer_tree_host_perftest"
   },
   "count": {
     "improvement_direction": "down",
     "why": "Processes"
   },
   "files": {
     "improvement_direction": "down",
     "why": "Static initializers"
   },
   "fps": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "The faster the better. Used by scirra benchmark."
   },
   "frames": {
     "improvement_direction": "down",
-    "why": "Dropped frames"
+    "why": "Dropped frames."
   },
   "frames-per-second": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Synonym for fps."
   },
   "frame_count": {
     "improvement_direction": "up",
     "why": "layer_tree_host_perftest"
   },
   "frame_time": {
     "improvement_direction": "down"
   },
   "kb": {
-    "improvement_direction": "down"
+    "improvement_direction": "down",
+    "why": "Synonym for KB, used in memory and io metrics."
   },
   "KB": {
-    "improvement_direction": "down"
+    "improvement_direction": "down",
+    "why": "KB of memory usage. Less memory usage is better. Used in endure."
   },
   "lines": {
     "improvement_direction": "up",
-    "why": "Coverage"
+    "why": "Coverage. More test coverage is better."
   },
   "load": {
     "improvement_direction": "down"
   },
   "MB": {
     "improvement_direction": "down"
   },
   "ms": {
-    "improvement_direction": "down"
+    "improvement_direction": "down",
+    "why": "Used in many Telemetry measurements. Fewer ms of time means faster."
   },
   "milliseconds": {
-    "improvement_direction": "down"
+    "improvement_direction": "down",
+    "why": "Synonym for ms."
   },
   "milliseconds-per-frame": {
     "improvement_direction": "down"
   },
   "objects (bigger is better)": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Used in spaceport benchmark."
   },
   "ObjectsAt30FPS": {
     "improvement_direction": "up"
   },
   "percent": {
-    "improvement_direction": "down"
+    "improvement_direction": "down",
+    "why": "Synonym for %, used in memory metric for percent fragmentation."
+  },
+  "coverage%": {
+    "improvement_direction": "up",
+    "why": "Used in alloy-perf-test/cts%/passed."
   },
   "points": {
     "improvement_direction": "up",
-    "why": "Synonym for score, used in ChromeOS touchpad tests"
+    "why": "Synonym for score, used in ChromeOS touchpad tests."
   },
   "ports": {
     "improvement_direction": "down"
   },
   "relocs": {
     "improvement_direction": "down"
   },
   "runs/ms": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Higher runs/ms implies faster execution."
   },
   "runs/s": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Used in dromaeo. Higher runs/s implies faster execution."
+  },
+  "runs_per_s": {
+    "improvement_direction": "up",
+    "why": "Synonym for runs/s, used in dromaeo data sent by cros bots."
   },
   "runs_per_second": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Synonym for runs/s."
   },
   "score": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Used in a variety of benchmarks where a higher score is better."
   },
   "score_(bigger_is_better)": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Synonym for score."
   },
   "score (bigger is better)": {
-    "improvement_direction": "up"
+    "improvement_direction": "up",
+    "why": "Synonym for score, used in jsgamebench and dom_perf."
   },
   "sec": {
     "improvement_direction": "down"
   },
   "seconds": {
     "improvement_direction": "down"
   },
   "us": {
     "improvement_direction": "down"
   }
 }
-