Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

libcef/browser/extensions/extension_web_contents_observer.h

@@ -5,8 +5,13 @@
 #ifndef CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_WEB_CONTENTS_OBSERVER_H_
 #define CEF_LIBCEF_BROWSER_EXTENSIONS_EXTENSION_WEB_CONTENTS_OBSERVER_H_
 
+#include <memory>
+
+#include "base/observer_list.h"
 #include "content/public/browser/web_contents_user_data.h"
 #include "extensions/browser/extension_web_contents_observer.h"
+#include "extensions/browser/script_execution_observer.h"
+#include "extensions/browser/script_executor.h"
 
 namespace extensions {
 
@@ -17,6 +22,8 @@ class CefExtensionWebContentsObserver
  public:
   ~CefExtensionWebContentsObserver() override;
 
+  ScriptExecutor* script_executor() { return script_executor_.get(); }
+
  private:
   friend class content::WebContentsUserData<CefExtensionWebContentsObserver>;
 
@@ -25,6 +32,12 @@ class CefExtensionWebContentsObserver
   // content::WebContentsObserver overrides.
   void RenderViewCreated(content::RenderViewHost* render_view_host) override;
 
+  // Our content script observers. Declare at top so that it will outlive all
+  // other members, since they might add themselves as observers.
+  base::ObserverList<ScriptExecutionObserver> script_execution_observers_;
+
+  std::unique_ptr<ScriptExecutor> script_executor_;
+
   DISALLOW_COPY_AND_ASSIGN(CefExtensionWebContentsObserver);
 };