Fix WebKit iOS build 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>