Revert 132312 - Proper fix for https://chromiumcodereview.appspot.com/10073005 :

Revert 132312 - Proper fix for https://chromiumcodereview.appspot.com/10073005 :

-----------------
Reason: build failure on Chromiumos Clang
-----------------

In file included from chrome/browser/chromeos/gdata/gdata_file_system_unittest.cc:27:
./chrome/browser/chromeos/gdata/mock_directory_change_observer.h:17:3:error: [chromium-style] Complex constructor has an inlined body.
  MockDirectoryChangeObserver() {}
  ^
./chrome/browser/chromeos/gdata/mock_directory_change_observer.h:18:3:error: [chromium-style] Complex destructor has an inline body.
  virtual ~MockDirectoryChangeObserver() {}
  ^
2 errors generated.

-----------------

gdata: Remove OnDirectoryChanged from MockGDataSyncClient,
added MockDirectoryChangeObserver for directory change notifications. 

BUG=none
TEST=existing GDataFileSystemTest.*


Review URL: http://codereview.chromium.org/10081032

TBR=zelidrag@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=132315