Add support for configurable -OX optimization levels for debug builds

tools/gn.py

Index: tools/gn.py
diff --git a/tools/gn.py b/tools/gn.py
index 1e5120b927b06d68eece76e4d65f49bb156e50fa..2b0afe8d8a210927749ab6b5fa666e13935e47b5 100755
--- a/tools/gn.py
+++ b/tools/gn.py
@@ -202,6 +202,8 @@ def to_gn_args(args, mode, arch, target_os):
     gn_args['use_goma'] = False
     gn_args['goma_dir'] = None
 
+  gn_args['dart_debug_optimization_level'] = args.debug_opt_level
+
   return gn_args
 
 
@@ -306,6 +308,11 @@ def parse_args(args):
       help='Use Clang',
       default=True,
       action='store_true')
+  other_group.add_argument('--debug-opt-level',
+      '-d',
+      help='The optimization level to use for debug builds',
+      type=int,
+      default=2)
   other_group.add_argument('--no-clang',
       help='Disable Clang',
       dest='clang',