Add is_apple (= is_mac || is_ios) to our GN files

Unblocks an incoming ANGLE roll

Change-Id: I7b1c69cda26cead4e9656c3da969c7b3263aaf14
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/353041
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

gn/BUILDCONFIG.gn

@@ -65,6 +65,9 @@ is_win = current_os == "win"
 # for ChromeOS, so this variable will not affect Skia proper.
 is_chromeos = false
 
+# This is to make the ANGLE build files happy. Skia always uses is_mac and/or is_ios.
+is_apple = is_mac || is_ios
+
 if (target_cpu == "") {
   target_cpu = host_cpu
   if (is_android || is_ios) {