Update to Chromium revision 12bfc336 (#338390).

- The ffmpeg library is now statically linked (see https://codereview.chromium.org/1141703002).
- Off-screen rendering of the PDF viewer does not work in combination with surfaces. Pass the
  `--disable-surfaces` command-line flag if GPU is enabled (see https://codereview.chromium.org/1169983006).

libcef/browser/chrome_browser_process_stub.h

@@ -96,6 +96,7 @@ class ChromeBrowserProcessStub : public BrowserProcess {
 #endif
   network_time::NetworkTimeTracker* network_time_tracker() override;
   gcm::GCMDriver* gcm_driver() override;
+  memory::OomPriorityManager* GetOomPriorityManager() override;
 
  private:
   std::string locale_;