Adding api option to download test data before a perf run.

scripts/slave/recipe_modules/v8/example.expected/forced_build.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "Fake_Master",
       "--builder",
       "Fake Builder",
@@ skipping 48 matching lines @@
     "cwd": "[SLAVE_BUILD]/v8",
     "name": "Example1",
     "~followup_annotations": [
       "@@@STEP_LINK@Results Dashboard@https://chrome-perf.googleplex.com/report?rev=12345&masters=Fake_Master&tests=v8%2FExample1&bots=ia32@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
+      "[SLAVE_BUILD]/v8/tools/run-test.py",
+      "--download-data-only",
+      "foo"
+    ],
+    "cwd": "[SLAVE_BUILD]/v8",
+    "name": "Example2 - download-data"
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
       "[SLAVE_BUILD]/v8/tools/run_perf.py",
       "--arch",
       "x64",
       "--buildbot",
       "--json-test-results",
       "/path/to/tmp/json",
       "example2.json",
       "--extra-flags=\"--flag1 --flag2\""
     ],
     "cwd": "[SLAVE_BUILD]/v8",
@@ skipping 32 matching lines @@
   },
   {
     "cmd": [
       "echo",
       "the result"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "do something with the results"
   }
 ]