WebKit's iOS and Mac builds do conditional compilation with preprocessor
directives rather than relying on the build system. Cocoa.h is not available on
iOS, so these Objective-C files must be conditionally compiled.

Bug: angleproject:3439
Change-Id: I8a1228f5b14ca6441c7d9a7f1c3f45d060003135
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1785653
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>