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.

include/capi/cef_resource_bundle_handler_capi.h

@@ -46,8 +46,9 @@ extern "C" {
 
 
 ///
-// Structure used to implement a custom resource bundle structure. The functions
-// of this structure may be called on multiple threads.
+// Structure used to implement a custom resource bundle structure. See
+// CefSettings for additional options related to resource bundle loading. The
+// functions of this structure may be called on multiple threads.
 ///
 typedef struct _cef_resource_bundle_handler_t {
   ///
@@ -56,26 +57,38 @@ typedef struct _cef_resource_bundle_handler_t {
   cef_base_t base;
 
   ///
-  // Called to retrieve a localized translation for the string specified by
-  // |message_id|. To provide the translation set |string| to the translation
-  // string and return true (1). To use the default translation return false
-  // (0). Supported message IDs are listed in cef_pack_strings.h.
+  // Called to retrieve a localized translation for the specified |string_id|.
+  // To provide the translation set |string| to the translation string and
+  // return true (1). To use the default translation return false (0). Include
+  // cef_pack_strings.h for a listing of valid string ID values.
   ///
   int (CEF_CALLBACK *get_localized_string)(
-      struct _cef_resource_bundle_handler_t* self, int message_id,
+      struct _cef_resource_bundle_handler_t* self, int string_id,
       cef_string_t* string);
 
   ///
-  // Called to retrieve data for the resource specified by |resource_id|. To
-  // provide the resource data set |data| and |data_size| to the data pointer
+  // Called to retrieve data for the specified scale independent |resource_id|.
+  // To provide the resource data set |data| and |data_size| to the data pointer
   // and size respectively and return true (1). To use the default resource data
   // return false (0). The resource data will not be copied and must remain
-  // resident in memory. Supported resource IDs are listed in
-  // cef_pack_resources.h.
+  // resident in memory. Include cef_pack_resources.h for a listing of valid
+  // resource ID values.
   ///
   int (CEF_CALLBACK *get_data_resource)(
       struct _cef_resource_bundle_handler_t* self, int resource_id, void** data,
       size_t* data_size);
+
+  ///
+  // Called to retrieve data for the specified |resource_id| nearest the scale
+  // factor |scale_factor|. To provide the resource data set |data| and
+  // |data_size| to the data pointer and size respectively and return true (1).
+  // To use the default resource data return false (0). The resource data will
+  // not be copied and must remain resident in memory. Include
+  // cef_pack_resources.h for a listing of valid resource ID values.
+  ///
+  int (CEF_CALLBACK *get_data_resource_for_scale)(
+      struct _cef_resource_bundle_handler_t* self, int resource_id,
+      cef_scale_factor_t scale_factor, void** data, size_t* data_size);
 } cef_resource_bundle_handler_t;