ScrollbarThemeComposite depends on Page/FrameView/ChromeClient
https://bugs.webkit.org/show_bug.cgi?id=21361

Reviewed by Andreas Kling.

Remove unused header includes.

* platform/ScrollbarThemeComposite.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148967 268f45cc-cd09-0410-ab3c-d52691b4dbfc