Add GetURLForBrowserTab method to the external file system backend.

chrome/browser/chromeos/fileapi/file_system_backend.cc

Index: chrome/browser/chromeos/fileapi/file_system_backend.cc
diff --git a/chrome/browser/chromeos/fileapi/file_system_backend.cc b/chrome/browser/chromeos/fileapi/file_system_backend.cc
index 5866c4721b5a1a21434032b0aee4e43190006240..c7329741abf7df6d197e6946b1cfef78c8fe6285 100644
--- a/chrome/browser/chromeos/fileapi/file_system_backend.cc
+++ b/chrome/browser/chromeos/fileapi/file_system_backend.cc
@@ -387,4 +387,29 @@ bool FileSystemBackend::GetVirtualPath(
          system_mount_points_->GetVirtualPath(filesystem_path, virtual_path);
 }
 
+void FileSystemBackend::GetAlternativeURL(
+    const storage::FileSystemURL& url,
+    base::Callback<void(const GURL& url)> callback) {
+
+  switch (url.type()) {
+    case storage::kFileSystemTypeDrive:
+      drive_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeProvided:
+      file_system_provider_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeDeviceMediaAsFileStorage:
+      mtp_delegate_->GetAlternativeURL(url, callback);
+      return;
+    case storage::kFileSystemTypeNativeLocal:
+    case storage::kFileSystemTypeRestrictedNativeLocal:
+      callback.Run(GURL());
+      return;
+    default:
+      NOTREACHED();
+      callback.Run(GURL());
+      return;
+  }
+}
+
 }  // namespace chromeos