[iOS] Purge GraphicsServices font cache on memory warning.

[iOS] Purge GraphicsServices font cache on memory warning.
<https://webkit.org/b/154343>

Reviewed by Antti Koivisto.

Source/WebCore:

The GS font cache was retaining CSS fonts after we stopped using them.
Call SPI to release them on memory pressure. This is one of multiple
steps necessary to actually free the fonts.

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

WebKitLibraries:

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

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@210688 268f45cc-cd09-0410-ab3c-d52691b4dbfc