[apple] Add a `is_apple` gn variable and use it

As https://crrev.com/c/2299189 adds `OS_MAC` & `OS_APPLE`, add a gn
variable `is_apple` that is true when `OS_APPLE` is defined. This
allow an easy mapping between gn variables and cpp macros.

Convert all usages of `is_ios || is_mac` or `is_mac || is_ios` to
use the new variable instead.

Bug: 1105907
Change-Id: I4cef55496a0ccf0a6162e60409ff3b7996f633dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315695
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#791803}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fafffe8843d5dac1ef479ae2e9058e0f93faf82d