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

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "Fake_Master",
       "--builder",
       "Fake Builder",
@@ skipping 52 matching lines @@
       "@@@STEP_LOG_LINE@Errors@Error line 1.@@@",
       "@@@STEP_LOG_LINE@Errors@Error line 2.@@@",
       "@@@STEP_LOG_END@Errors@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "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 13 matching lines @@
     "cwd": "[SLAVE_BUILD]",
     "name": "perf dashboard post",
     "stdin": "{\"data\": [{\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"8.16496580928\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/Richards\", \"units\": \"score\", \"value\": \"20.0\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"0.0\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/DeltaBlue\", \"units\": \"ms\", \"value\": \"1.2\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"8.16496580928\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/Richards\", \"units\": \"score\", \"value\": \"20.0\"}, {\"bot\": \"ia32\", \"buildername\": \"Fake Builder\", \"buildnumber\": 571, \"error\": \"0.0\", \"master\": \"Fake_Master\", \"masterid\": \"Fake_Master\", \"revision\": \"12345\", \"supplemental_columns\": {\"a_default_rev\": \"r_v8_git\", \"r_v8_git\": \"deadbeef\"}, \"test\": \"v8/v8/DeltaBlue\", \"units\": \"ms\", \"value\": \"1.2\"}], \"url\": \"https://chrome-perf.googleplex.com/add_point\"}"
   },
   {
     "name": "$final_result",
     "reason": "One or more performance tests failed.",
     "status_code": 1
   }
 ]