[chromium] Add WebPluginScrollbar and WebPluginScrollbarClient stub implementations

[chromium] Add WebPluginScrollbar and WebPluginScrollbarClient stub implementations
https://bugs.webkit.org/show_bug.cgi?id=89928

Reviewed by Darin Fisher.

These classes are just typedef'd to be the non-Plugin version. The
next step is to change Chromium to use the Plugin version, then move
most of WebPluginScrollbar to a WebScrollbar in Platform to separate
out the base functionality, while providing the same top level API.

* WebKit.gyp:
* public/WebPluginScrollbar.h: Added.
(WebKit):
* public/WebPluginScrollbarClient.h: Added.
(WebKit):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@121227 268f45cc-cd09-0410-ab3c-d52691b4dbfc