deleteBackingStore() from freeing the bitmap while it is still being
referenced by Core Animation.

Reviewed by Maciej Stachowiak.

https://bugs.webkit.org/show_bug.cgi?id=37954

* WebView.cpp:
(WebView::ensureBackingStore):
(WebView::addToDirtyRegion):
(WebView::scrollBackingStore):
(WebView::updateBackingStore):
(WebView::paint):
(WebView::backingStore):
(releaseBackingStoreCallback): deref m_backingStoreBitmap once Core
Animation has dropeed its reference to the memory.
(WebView::updateRootLayerContents): ref m_backingStoreBitmap before
passing the memory to Core Animation to prevent deleteBackingStore()
from freeing it while it is still referenced by CA.
* WebView.h: Make m_backingStoreBitmap a RefCountedGDIHandle<HBITMAP>,
and make m_backingStoreDirtyRegion a RefCountedGDIHandle<HRGN>.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58067 268f45cc-cd09-0410-ab3c-d52691b4dbfc