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.

tests/cefclient/browser/root_window.cc

@@ -9,10 +9,36 @@
 
 namespace client {
 
+RootWindowConfig::RootWindowConfig()
+    : with_controls(true),
+      with_osr(false),
+      with_extension(false),
+      initially_hidden(false),
+      url(MainContext::Get()->GetMainURL()) {}
+
+RootWindow::RootWindow() : delegate_(NULL) {}
+
+RootWindow::~RootWindow() {}
+
 // static
 scoped_refptr<RootWindow> RootWindow::GetForBrowser(int browser_id) {
   return MainContext::Get()->GetRootWindowManager()->GetWindowForBrowser(
       browser_id);
 }
 
+void RootWindow::OnExtensionsChanged(const ExtensionSet& extensions) {
+  REQUIRE_MAIN_THREAD();
+  DCHECK(delegate_);
+  DCHECK(!WithExtension());
+
+  if (extensions.empty())
+    return;
+
+  ExtensionSet::const_iterator it = extensions.begin();
+  for (; it != extensions.end(); ++it) {
+    delegate_->CreateExtensionWindow(*it, CefRect(), NULL, base::Closure(),
+                                     WithWindowlessRendering());
+  }
+}
+
 }  // namespace client