Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

libcef/browser/browser_context_proxy.cc

@@ -169,3 +169,11 @@ net::URLRequestContextGetter*
         content::URLRequestInterceptorScopedVector request_interceptors) {
   return NULL;
 }
+
+HostContentSettingsMap* CefBrowserContextProxy::GetHostContentSettingsMap() {
+  return parent_->GetHostContentSettingsMap();
+}
+
+PrefService* CefBrowserContextProxy::GetPrefs() {
+  return parent_->GetPrefs();
+}