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/common/cef_switches.cc

@@ -116,4 +116,14 @@ const char kWidevineCdmPath[]             = "widevine-cdm-path";
 // Widevine CDM version.
 const char kWidevineCdmVersion[]          = "widevine-cdm-version";
 
+// Default plugin policy action.
+const char kPluginPolicy[]                = "plugin-policy";
+// Allow the content. This is the default value.
+const char kPluginPolicy_Allow[]          = "allow";
+// Allow important content and block unimportant content based on heuristics.
+// The user can manually load blocked content.
+const char kPluginPolicy_Detect[]         = "detect";
+// Block the content. The user can manually load blocked content.
+const char kPluginPolicy_Block[]          = "block";
+
 }  // namespace switches