[BlackBerry] DrawingBuffer: add clearPlatformLayer() implementation

[BlackBerry] DrawingBuffer: add clearPlatformLayer() implementation
https://bugs.webkit.org/show_bug.cgi?id=112696

Patch by Alberto Garcia <agarcia@igalia.com> on 2013-03-21
Reviewed by Rob Buis.

DrawingBuffer::clearPlatformLayer() was added in r139142.

This is not needed in the BlackBerry port, so this patch only
contains a dummy implementation.

* platform/graphics/blackberry/DrawingBufferBlackBerry.cpp:
(WebCore):
(WebCore::DrawingBuffer::clearPlatformLayer):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@146497 268f45cc-cd09-0410-ab3c-d52691b4dbfc