FBTF: Move the TabRestoreService::Observer into its own file.

chrome/browser/sessions/session_backend.h

Index: chrome/browser/sessions/session_backend.h
diff --git a/chrome/browser/sessions/session_backend.h b/chrome/browser/sessions/session_backend.h
index 523ef58b7b0a124e4b50ba40b6a4b77a74347c13..883a58f4c0e315e197450ad507b5ae4d43bac1ff 100644
--- a/chrome/browser/sessions/session_backend.h
+++ b/chrome/browser/sessions/session_backend.h
@@ -12,7 +12,10 @@
 #include "base/scoped_ptr.h"
 #include "chrome/browser/sessions/base_session_service.h"
 #include "chrome/browser/sessions/session_command.h"
-#include "net/base/file_stream.h"
+
+namespace net {
+class FileStream;
+}
 
 // SessionBackend -------------------------------------------------------------
 
@@ -93,7 +96,7 @@ class SessionBackend : public base::RefCountedThreadSafe<SessionBackend> {
  private:
   friend class base::RefCountedThreadSafe<SessionBackend>;
 
-  ~SessionBackend() {}
+  ~SessionBackend();
 
   // If current_session_file_ is open, it is truncated so that it is essentially
   // empty (only contains the header). If current_session_file_ isn't open, it