[iOS/Mac/GN] Update timestamp when copying a tree.

build/toolchain/mac/compile_xcassets.py

Index: build/toolchain/mac/compile_xcassets.py
diff --git a/build/toolchain/mac/compile_xcassets.py b/build/toolchain/mac/compile_xcassets.py
index b78854416bab5852420995b49ff181cef29f3544..ca51340fbc27b46ef67066b2d2b80f899437c093 100644
--- a/build/toolchain/mac/compile_xcassets.py
+++ b/build/toolchain/mac/compile_xcassets.py
@@ -11,6 +11,11 @@ def Main():
   parser = argparse.ArgumentParser(
       description='compile assets catalog for a bundle')
   parser.add_argument(
+      '--version-for-gn', choices=('1',),
+      help='version need to be increased in this script and in the file '
+           '//build/toolchain/mac/BUILD.gn to force build until issue '
+           'http://crbug.com/619083 is fixed')
+  parser.add_argument(
       '--platform', '-p', required=True,
       choices=('macosx', 'iphoneos', 'iphonesimulator'),
       help='target platform for the compiled assets catalog')
@@ -30,6 +35,9 @@ def Main():
         'output should be path to compiled asset catalog, not '
         'to the containing bundle: %s\n' % (args.output,))
 
+  if os.path.exists(args.output):
+    os.unlink(args.output)
+
   command = [
       'xcrun', 'actool', '--output-format', 'human-readable-text',
       '--compress-pngs', '--notices', '--warnings', '--errors',