Add GetURLForBrowserTab method to the external file system backend.

webkit/browser/fileapi/file_system_backend.h

Index: webkit/browser/fileapi/file_system_backend.h
diff --git a/webkit/browser/fileapi/file_system_backend.h b/webkit/browser/fileapi/file_system_backend.h
index b71c8cab06d7fce114b06a3ade098dff8084f165..29bbe4a2c4da8576ab36768f1fa6d6aeeff6d3ff 100644
--- a/webkit/browser/fileapi/file_system_backend.h
+++ b/webkit/browser/fileapi/file_system_backend.h
@@ -47,6 +47,7 @@ class STORAGE_EXPORT FileSystemBackend {
                               const std::string& name,
                               base::File::Error error)>
       OpenFileSystemCallback;
+  typedef base::Callback<void(const GURL& url)> URLCallback;
   virtual ~FileSystemBackend() {}
 
   // Returns true if this filesystem backend can handle |type|.
@@ -157,6 +158,11 @@ class ExternalFileSystemBackend : public FileSystemBackend {
   // path is not exposed by this provider.
   virtual bool GetVirtualPath(const base::FilePath& file_system_path,
                               base::FilePath* virtual_path) = 0;
+  // Gets an redirect URL for browser tab. e.g. Google Drive URL for hosted
+  // documents. Returns empty URL if the entry does not have the redirect URL.
+  virtual void GetURLForBrowserTab(
+      const storage::FileSystemURL& url,
+      const URLCallback& callback) = 0;
 };
 
 }  // namespace storage