Address some review comments noted in r158463
https://bugs.webkit.org/show_bug.cgi?id=123646

Reviewed by Darin Adler.

* Shared/mac/RemoteLayerBackingStore.mm:
Put our __has_include ifs all on one line.

(createIOSurface):
IOSurfaceAlignProperty returns size_t, not unsigned long.

(RemoteLayerBackingStore::display):
(RemoteLayerBackingStore::drawInContext):
Create the frontImage in display() and hand it to drawInContext().

Use the GraphicsContext API instead of CG itself for clearRect().

When using ShareableBitmap, replace m_frontBuffer instead of
having a backBuffer local; the CGImageRef will retain the
ShareableBitmap it references.

Duplicate our drawInContext() calls so we don't have to allocate
a GraphicsContext on the heap in the IOSurface case.

Don't bother clearing frontContext; it'll happen for us at
the appropriate time. Instead, assert that it's still alive after
we drop our reference to the frontImage created from it.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158488 268f45cc-cd09-0410-ab3c-d52691b4dbfc