Pass rietveld as the default patch_storage.

Passing around empty strings as command line arguments requires
us to be really careful about command line quoting, parsing, etc.

It seems easy enough here to just supply the implied value.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2323233003

Review-Url: https://codereview.chromium.org/2323233003

infra/bots/recipe_modules/vars/api.py

@@ -108,7 +108,7 @@ class SkiaVarsApi(recipe_api.RecipeApi):
     self.default_env.update({'SKIA_OUT': self.skia_out,
                              'BUILDTYPE': self.configuration})
     self.is_trybot = self.builder_cfg['is_trybot']
-    self.patch_storage = self.m.properties.get('patch_storage', '')
+    self.patch_storage = self.m.properties.get('patch_storage', 'rietveld')
     self.issue = None
     self.patchset = None
     if self.is_trybot:

infra/bots/recipes/swarm_perf.expected/Perf-Win8-MSVC-ShuttleB-GPU-HD4600-x86_64-Release-Trybot.json

@@ -167,7 +167,7 @@
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--key",
       "arch",
       "x86_64",

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

@@ -167,7 +167,7 @@
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--key",
       "arch",
       "x86_64",

infra/bots/recipes/swarm_test.expected/Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Coverage-Trybot.json

@@ -148,7 +148,7 @@
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--svgs",
       "[SLAVE_BUILD]/svg",
       "--key",
@@ -447,7 +447,7 @@
       "patchset",
       "1",
       "patch_storage",
-      ""
+      "rietveld"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {

infra/bots/recipes/swarm_test.expected/Test-Win8-MSVC-ShuttleB-CPU-AVX2-x86_64-Release-Trybot.json

@@ -200,7 +200,7 @@
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--svgs",
       "[SLAVE_BUILD]\\svg",
       "--key",

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

@@ -200,7 +200,7 @@
       "patchset",
       "1",
       "patch_storage",
-      "",
+      "rietveld",
       "--svgs",
       "[SLAVE_BUILD]\\svg",
       "--key",