Enable Dart compressed pointers for 64-bit mobile targets (flutter#27975)

* Enable Dart compressed pointers for 64-bit mobile targets

* Fix.

* Address code review comments.

Author: a-siva

tools/gn

@@ -380,6 +380,10 @@ def to_gn_args(args):
     # on Android.
     gn_args['bssl_use_clang_integrated_as'] = True
 
+    # Enable pointer compression on 64-bit mobile targets.
+    if args.target_os in ['android', 'ios'] and gn_args['target_cpu'] in ['x64' , 'arm64']:
+      gn_args['dart_use_compressed_pointers'] = True
+
     return gn_args
 
 def parse_args(args):