Add GetURLForBrowserTab method to the external file system backend.

chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc

Index: chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc
diff --git a/chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc b/chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc
index a191e75f4e9f3b19e12e49205a8734264116145e..b1aa8ecf8820778f1eb333953c8bc3e0d85c0871 100644
--- a/chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc
+++ b/chrome/browser/chromeos/drive/fileapi/file_system_backend_delegate.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
+#include "chrome/browser/chromeos/drive/file_system_interface.h"
 #include "chrome/browser/chromeos/drive/file_system_util.h"
 #include "chrome/browser/chromeos/drive/fileapi/async_file_util.h"
 #include "chrome/browser/chromeos/drive/fileapi/fileapi_worker.h"
@@ -22,6 +23,46 @@
 using content::BrowserThread;
 
 namespace drive {
+namespace {
+void PostURLOnIOThread(
+    base::Callback<void(const GURL& url)> callback,
+    const GURL& url) {
+  BrowserThread::PostTask(
+      BrowserThread::IO,
+      FROM_HERE,
+      base::Bind(callback, url));
+}
+
+void GetAlternativeURLOnUIThreadWithResourceEntry(
+    base::Callback<void(const GURL& url)> callback,
+    FileError error,
+    scoped_ptr<ResourceEntry> entry) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  GURL url;
+  if (error == FILE_ERROR_OK &&
+      entry->has_file_specific_info() &&
+      entry->file_specific_info().is_hosted_document()) {
+    url = GURL(entry->file_specific_info().alternate_url());
+  }
+  PostURLOnIOThread(callback, url);
+}
+
+void GetAlternativeURLOnUIThread(
+    const storage::FileSystemURL& url,
+    base::Callback<void(const GURL& url)> callback) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  FileSystemInterface* const file_system =
+      fileapi_internal::GetFileSystemFromUrl(url);
+  if (!file_system) {
+    PostURLOnIOThread(callback, GURL());
+    return;
+  }
+  const base::FilePath file_path = util::ExtractDrivePathFromFileSystemUrl(url);
+  file_system->GetResourceEntry(
+      file_path,
+      base::Bind(GetAlternativeURLOnUIThreadWithResourceEntry, callback));
+}
+}  // namespace
 
 FileSystemBackendDelegate::FileSystemBackendDelegate()
     : async_file_util_(new internal::AsyncFileUtil) {
@@ -86,4 +127,14 @@ storage::WatcherManager* FileSystemBackendDelegate::GetWatcherManager(
   return NULL;
 }
 
+void FileSystemBackendDelegate::GetAlternativeURL(
+    const storage::FileSystemURL& url,
+    base::Callback<void(const GURL& url)> callback) {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  BrowserThread::PostTask(
+      BrowserThread::UI,
+      FROM_HERE,
+      base::Bind(GetAlternativeURLOnUIThread, url, callback));
+}
+
 }  // namespace drive