Update to Chromium revision 6e53600d (#386251)

- Remove |accept_lang| parameter from CefJSDialogHandler::OnJSDialog
  and CefFormatUrlForSecurityDisplay (see https://crbug.com/336973#c36).
- Remove remaining NPAPI-related code including functions from
  cef_web_plugin.h (see https://crbug.com/493212#c55).
- Mac: 10.7+ deployment target is now required for client applications.
- Mac: Remove CefBrowserHost::SetWindowVisibility (issue #1375). No
  replacement is required for windowed rendering. Use WasHidden for
  off-screen rendering.
- Windows: Visual Studio 2015 Update 2 is now required when building
  CEF/Chromium.

include/cef_browser.h

@@ -353,13 +353,6 @@ class CefBrowserHost : public virtual CefBase {
   /*--cef()--*/
   virtual void SetFocus(bool focus) =0;
 
-  ///
-  // Set whether the window containing the browser is visible
-  // (minimized/unminimized, app hidden/unhidden, etc). Only used on Mac OS X.
-  ///
-  /*--cef()--*/
-  virtual void SetWindowVisibility(bool visible) =0;
-
   ///
   // Retrieve the window handle for this browser. If this browser is wrapped in
   // a CefBrowserView this method should be called on the browser process UI