Move the chrome.automation API to dev channel.

BUG=309681,414153
TEST=AutomationApiTest.*.

Review URL: https://codereview.chromium.org/561933002

Cr-Commit-Position: refs/heads/master@{#294982}

chrome/common/extensions/api/_manifest_features.json

@@ -39,7 +39,7 @@
     "extension_types": "all"
   },
   "automation": {
-    "channel": "trunk",
+    "channel": "dev",
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "bluetooth": [{