mac: Make it a bit easier to run "gn gen" on different hosts.

Still far from working though.

Bug: 1147069
Change-Id: Ie6d519915076fe774c3a2af5fc4e089a6681eb2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526911
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825402}

build_overrides/build.gni

@@ -60,7 +60,7 @@ declare_args() {
   use_system_xcode = ""
 }
 
-if (host_os == "mac" && use_system_xcode == "") {
+if (is_apple && use_system_xcode == "") {
   _result = exec_script("//build/mac/should_use_hermetic_xcode.py",
                         [
                           "--xcode-version=$mac_xcode_version",
@@ -76,4 +76,5 @@ if (host_os == "mac" && use_system_xcode == "") {
              "Please install Xcode.")
 
   use_system_xcode = _result != 1
+  assert(host_os == "mac" || !use_system_xcode)
 }

tools/v8_context_snapshot/v8_context_snapshot.gni

@@ -13,11 +13,12 @@ import("//v8/gni/v8.gni")
 
 declare_args() {
   # TODO(crbug.com/764576): Enable the feature on more environments.
-  # Disable in win/cross builds since buliding Blink twice is slow.
+  # Disable in mac and win cross builds since building Blink twice is slow.
   use_v8_context_snapshot =
       !is_chromeos && !is_android && !is_chromecast && !is_fuchsia &&
       !(host_os == "mac" && current_cpu == "x86") &&
-      (v8_target_cpu == target_cpu || is_msan) && !(is_win && host_os != "win")
+      (v8_target_cpu == target_cpu || is_msan) &&
+      !(is_win && host_os != "win") && !(is_mac && host_os != "mac")
 
   # We use a different filename for arm64 macOS builds so that the arm64 and
   # x64 snapshots can live side-by-side in a universal macOS app.