[iOS WK2] Avoid creating tiles that are too large for rendering

[iOS WK2] Avoid creating tiles that are too large for rendering
https://bugs.webkit.org/show_bug.cgi?id=156050
rdar://problem/25424541

Reviewed by Darin Adler.

In the case of a WKWebView which is sized large enough to show an entire document
(for example, when enclosed inside a UIScrollView), we would create tiles that were
larger than CoreAnimation was willing to deal with.

* platform/graphics/ca/TileController.cpp:
(WebCore::TileController::tileSize): Take the device scale, and the max IOSurface size
into account when computing the max tile size.
* platform/graphics/cocoa/IOSurface.mm:
(IOSurface::maximumSize): CoreAnimation imposes limits in addition to the size reported
by IOSurfaceGetPropertyMaximum(), namely an 8k cap, so mimic that here.
(IOSurface::ensurePlatformContext): Typo.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@198875 268f45cc-cd09-0410-ab3c-d52691b4dbfc