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.h

@@ -104,6 +104,9 @@
 //    CefURLRequestContextGetter* destruction. 
 */
 
+class HostContentSettingsMap;
+class PrefService;
+
 namespace extensions {
 class CefExtensionSystem;
 }
@@ -142,6 +145,12 @@ class CefBrowserContext
       content::ProtocolHandlerMap* protocol_handlers,
       content::URLRequestInterceptorScopedVector request_interceptors) = 0;
 
+  // Settings for plugins and extensions.
+  virtual HostContentSettingsMap* GetHostContentSettingsMap() = 0;
+
+  // Preferences.
+  virtual PrefService* GetPrefs() = 0;
+
   CefResourceContext* resource_context() const {
     return resource_context_.get();
   }