[fsp] Enable the File System Provider API on the dev channel.

The read-only version is pretty much done, so we should let developers use it
on the dev channel for initial feedback.

TEST=browser_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/531463002

Cr-Commit-Position: refs/heads/master@{#293063}

chrome/browser/chromeos/extensions/file_system_provider/file_system_provider_apitest.cc

@@ -9,9 +9,7 @@ namespace extensions {
 class FileSystemProviderApiTest : public ExtensionApiTest {
  public:
   FileSystemProviderApiTest()
-      // Set the channel to "trunk" since this API is restricted to trunk.
-      : current_channel_(chrome::VersionInfo::CHANNEL_UNKNOWN) {
-  }
+      : current_channel_(chrome::VersionInfo::CHANNEL_DEV) {}
 
   // Loads a helper testing extension.
   virtual void SetUpOnMainThread() OVERRIDE {

chrome/common/extensions/api/_permission_features.json

@@ -558,7 +558,7 @@
     ]
   }],
   "fileSystemProvider": {
-    "channel": "trunk",
+    "channel": "dev",
     "extension_types": ["platform_app"]
   },
   "firstRunPrivate": {