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/internal/cef_types.h

@@ -1423,6 +1423,11 @@ typedef enum {
   MENU_ID_NO_SPELLING_SUGGESTIONS        = 205,
   MENU_ID_ADD_TO_DICTIONARY              = 206,
 
+  // Custom menu items originating from the renderer process. For example,
+  // plugin placeholder menu items or Flash menu items.
+  MENU_ID_CUSTOM_FIRST        = 220,
+  MENU_ID_CUSTOM_LAST         = 250,
+
   // All user-defined menu IDs should come between MENU_ID_USER_FIRST and
   // MENU_ID_USER_LAST to avoid overlapping the Chromium and CEF ID ranges
   // defined in the tools/gritsettings/resource_ids file.
@@ -2241,6 +2246,50 @@ typedef struct _cef_pdf_print_settings_t {
 
 } cef_pdf_print_settings_t;
 
+///
+// Supported UI scale factors for the platform. SCALE_FACTOR_NONE is used for
+// density independent resources such as string, html/js files or an image that
+// can be used for any scale factors (such as wallpapers).
+///
+typedef enum {
+  SCALE_FACTOR_NONE = 0,
+  SCALE_FACTOR_100P,
+  SCALE_FACTOR_125P,
+  SCALE_FACTOR_133P,
+  SCALE_FACTOR_140P,
+  SCALE_FACTOR_150P,
+  SCALE_FACTOR_180P,
+  SCALE_FACTOR_200P,
+  SCALE_FACTOR_250P,
+  SCALE_FACTOR_300P,
+} cef_scale_factor_t;
+
+///
+// Plugin policies supported by CefRequestContextHandler::OnBeforePluginLoad.
+///
+typedef enum {
+  ///
+  // Allow the content.
+  ///
+  PLUGIN_POLICY_ALLOW,
+
+  ///
+  // Allow important content and block unimportant content based on heuristics.
+  // The user can manually load blocked content.
+  ///
+  PLUGIN_POLICY_DETECT_IMPORTANT,
+
+  ///
+  // Block the content. The user can manually load blocked content.
+  ///
+  PLUGIN_POLICY_BLOCK,
+
+  ///
+  // Disable the content. The user cannot load disabled content.
+  ///
+  PLUGIN_POLICY_DISABLE,
+} cef_plugin_policy_t;
+
 #ifdef __cplusplus
 }
 #endif