Move plugin placeholder style to CSS, and allow it to bypass main world CSP.

Source/core/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 import("//third_party/WebKit/Source/bindings/bindings.gni")
 import("//third_party/WebKit/Source/bindings/core/v8/generated.gni")
 import("//third_party/WebKit/Source/bindings/modules/modules.gni")
 import("//third_party/WebKit/Source/bindings/modules/v8/generated.gni")
 import("//third_party/WebKit/Source/bindings/scripts/scripts.gni")
@@ skipping 1013 matching lines @@
     "css/themeWin.css",
     "css/themeWinQuirks.css",
     "css/svg.css",
     "css/navigationTransitions.css",
     "css/mathml.css",
     "css/mediaControls.css",
     "css/mediaControlsAndroid.css",
     "css/fullscreen.css",
     "css/xhtmlmp.css",
     "css/viewportAndroid.css",
+    "html/shadow/PluginPlaceholderElement.css",
   ]
 
   outputs = [
     "$blink_core_output_dir/UserAgentStyleSheets.h",
     "$blink_core_output_dir/UserAgentStyleSheetsData.cpp",
   ]
 
   args =
     [ "--namespace", "blink" ] +
     [ "--out-h=" + rebase_path(outputs[0], root_build_dir) ] +
@@ skipping 39 matching lines @@
     "$blink_core_output_dir/{{source_name_part}}.h",
   ]
   args = [
     "{{source}}",
     rel_blink_core_gen_dir,
     bison_exe,
   ]
 
   deps = make_core_generated_deps
 }