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/extensions/extensions_browser_client.cc

@@ -8,6 +8,7 @@
 #include "libcef/browser/content_browser_client.h"
 #include "libcef/browser/extensions/component_extension_resource_manager.h"
 #include "libcef/browser/extensions/extension_system_factory.h"
+#include "libcef/browser/extensions/extension_web_contents_observer.h"
 #include "libcef/browser/extensions/extensions_api_client.h"
 #include "libcef/browser/extensions/url_request_util.h"
 
@@ -226,6 +227,12 @@ bool CefExtensionsBrowserClient::IsMinBrowserVersionSupported(
   return true;
 }
 
+ExtensionWebContentsObserver*
+CefExtensionsBrowserClient::GetExtensionWebContentsObserver(
+    content::WebContents* web_contents) {
+  return CefExtensionWebContentsObserver::FromWebContents(web_contents);
+}
+
 void CefExtensionsBrowserClient::SetAPIClientForTest(
     ExtensionsAPIClient* api_client) {
   api_client_.reset(api_client);