Make chromium canary recipe explicitly set GIT_MODE

This is completely safe because this recipe isn't
used for anything.

TBR=iannucci@chromium.org
BUG=640251

Review URL: https://codereview.chromium.org/2276033004 .

Cr-Commit-Position: refs/heads/master@{#414275}

infra/recipes/canary.expected/basic.json

@@ -11,7 +11,7 @@
       "--slave",
       "TestSlavename",
       "--spec",
-      "cache_dir = '[GIT_CACHE]'\nsolutions = [{'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'svn://svn-mirror.golo.chromium.org/blink/trunk'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn-mirror.golo.chromium.org/chrome/trunk/src'}]",
+      "cache_dir = '[GIT_CACHE]'\nsolutions = [{'custom_vars': {'googlecode_url': 'svn://svn-mirror.golo.chromium.org/%s', 'nacl_trunk': 'svn://svn-mirror.golo.chromium.org/native_client/trunk', 'sourceforge_url': 'svn://svn-mirror.golo.chromium.org/%(repo)s', 'webkit_trunk': 'https://chromium.googlesource.com/chromium/blink.git'}, 'deps_file': '.DEPS.git', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}]",
       "--root",
       "src",
       "--revision_mapping_file",

infra/recipes/canary.py

@@ -11,7 +11,7 @@ DEPS = [
 ]
 
 def RunSteps(api):
-  api.gclient.set_config('chromium')
+  api.gclient.set_config('chromium', GIT_MODE=True)
   api.bot_update.ensure_checkout(force=True)
 
 def GenTests(api):