Revert of Makes ScrollbarTheme call through to theme engine for corner (https://codereview.chromium.org/149093009/)

Reason for revert:
Broke Android Tests (dbg)/webkit_unittests:

[FATAL:webthemeengine_impl_android.cc(37)] Check failed: false. 

http://build.chromium.org/p/chromium.webkit/builders/Android%20Tests%20%28dbg%29

Original issue's description:
> Makes ScrollbarTheme call through to theme engine for corner
> 
> Hopefully the last bit of wiring needed to let the theme engine draw
> the corner.
> 
> BUG=333499
> TEST=none
> R=jamesr@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=166511

TBR=jamesr@chromium.org,sky@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=333499

Review URL: https://codereview.chromium.org/156163004

git-svn-id: svn://svn.chromium.org/blink/trunk@166552 bbb929c8-8fbe-4397-9dbb-9b2b20218538