Remove WebContents::InsertCSS since it is unused

content/common/frame_messages.h

 // Copyright 2013 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.
 
 // IPC messages for interacting with frames.
 // Multiply-included message file, hence no include guard.
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 686 matching lines @@
 IPC_MESSAGE_ROUTED0(FrameMsg_Stop)
 
 // A message sent to RenderFrameProxy to indicate that its corresponding
 // RenderFrame has started loading a document.
 IPC_MESSAGE_ROUTED0(FrameMsg_DidStartLoading)
 
 // A message sent to RenderFrameProxy to indicate that its corresponding
 // RenderFrame has completed loading.
 IPC_MESSAGE_ROUTED0(FrameMsg_DidStopLoading)
 
-// Request for the renderer to insert CSS into the frame.
-IPC_MESSAGE_ROUTED1(FrameMsg_CSSInsertRequest,
-                    std::string  /* css */)
-
 // Add message to the frame console.
 IPC_MESSAGE_ROUTED2(FrameMsg_AddMessageToConsole,
                     content::ConsoleMessageLevel /* level */,
                     std::string /* message */)
 
 // Request for the renderer to execute JavaScript in the frame's context.
 //
 // javascript is the string containing the JavaScript to be executed in the
 // target frame's context.
 //
@@ skipping 858 matching lines @@
 // nearest find result in the sending frame.
 IPC_MESSAGE_ROUTED2(FrameHostMsg_GetNearestFindResult_Reply,
                     int /* nfr_request_id */,
                     float /* distance */)
 #endif
 
 // Adding a new message? Stick to the sort order above: first platform
 // independent FrameMsg, then ifdefs for platform specific FrameMsg, then
 // platform independent FrameHostMsg, then ifdefs for platform specific
 // FrameHostMsg.