[ios] Set `target_environment` gn variable in setup-gn.py

In preparation of supporting arm64 simulator build (for Apple Silicon
macs), we need to decouple the fact that a build is a simulator build
from the `target_cpu`.

This was done by introducing a `target_environment` variable that can
be set by the user (the default value is set using the same heuristic
based on value of `target_cpu` if unset).

Configure setup-gn.py to be explicit about setting the variable.

Bug: 1138425
Change-Id: Ic8f84ef260972e542ea456d6e85e43080e81da36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489984
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819768}

ios/build/tools/setup-gn.py

@@ -72,6 +72,11 @@ class GnGenerator(object):
     }
   }
 
+  TARGET_ENVIRONMENT_VALUES = {
+    'iphoneos': '"device"',
+    'iphonesimulator': '"simulator"',
+  }
+
   def __init__(self, settings, config, target):
     assert target in SUPPORTED_TARGETS
     assert config in SUPPORTED_CONFIGS
@@ -103,7 +108,7 @@ class GnGenerator(object):
     args.append(('enable_stripping', 'enable_dsyms'))
     args.append(('is_official_build', self._config == 'Official'))
     args.append(('is_chrome_branded', 'is_official_build'))
-    args.append(('use_xcode_clang', 'false'))
+    args.append(('use_xcode_clang', False))
     args.append(('use_clang_coverage', self._config == 'Coverage'))
     args.append(('is_component_build', False))
 
@@ -120,6 +125,10 @@ class GnGenerator(object):
     else:
       args.append(('target_cpu', cpu_values[build_arch]))
 
+    args.append((
+        'target_environment',
+        self.TARGET_ENVIRONMENT_VALUES[self._target]))
+
     # Add user overrides after the other configurations so that they can
     # refer to them and override them.
     args.extend(self._settings.items('gn_args'))