Pass rietveld as the default patch_storage.

infra/bots/recipes/swarm_test.expected/big_issue_number.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]\\skia\\infra\\bots\\assets\\skp\\VERSION",
       "/path/to/tmp/"
     ],
     "name": "Get downloaded SKP VERSION"
@@ skipping 182 matching lines @@
       "client.skia.compile",
       "builder",
       "Test-Win8-MSVC-ShuttleB-CPU-AVX2-x86_64-Release-Trybot",
       "build_number",
       "5",
       "issue",
       "2147533002",
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--svgs",
       "[SLAVE_BUILD]\\svg",
       "--key",
       "arch",
       "x86_64",
       "compiler",
       "MSVC",
       "configuration",
       "Release",
       "cpu_or_gpu",
@@ skipping 306 matching lines @@
       "SKIA_OUT": "[SLAVE_BUILD]\\out"
     },
     "name": "dm"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]