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.

tests/cefclient/browser/client_app_browser.cc

@@ -11,6 +11,7 @@
 namespace client {
 
 ClientAppBrowser::ClientAppBrowser() {
+  CreateDelegates(delegates_);
 }
 
 void ClientAppBrowser::OnBeforeCommandLineProcessing(
@@ -46,12 +47,14 @@ void ClientAppBrowser::OnBeforeCommandLineProcessing(
       // See https://bitbucket.org/chromiumembedded/cef/issues/1368 for details.
       command_line->AppendSwitch("enable-begin-frame-scheduling");
     }
+
+    DelegateSet::iterator it = delegates_.begin();
+    for (; it != delegates_.end(); ++it)
+      (*it)->OnBeforeCommandLineProcessing(this, command_line);
   }
 }
 
 void ClientAppBrowser::OnContextInitialized() {
-  CreateDelegates(delegates_);
-
   // Register cookieable schemes with the global cookie manager.
   CefRefPtr<CefCookieManager> manager =
       CefCookieManager::GetGlobalManager(NULL);