[iOS] Ensure that GraphicsServices is initialized before calling GSFontPurgeFontCache().

[iOS] Ensure that GraphicsServices is initialized before calling GSFontPurgeFontCache().
https://bugs.webkit.org/show_bug.cgi?id=173616
<rdar://problem/30780050>

Reviewed by Chris Dumez.

Source/WebCore:

Fix for crash in GSFontPurgeFontCache() seen in apps embedding WebKit.
Ensure GSFontInitialize() has been called first, since the former depends on state
set up by this function.

* page/cocoa/MemoryReleaseCocoa.mm:
(WebCore::platformReleaseMemory):
* platform/spi/ios/GraphicsServicesSPI.h:

WebKitLibraries:

* WebKitPrivateFrameworkStubs/iOS/10/GraphicsServices.framework/GraphicsServices.tbd:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@218643 268f45cc-cd09-0410-ab3c-d52691b4dbfc