Fix test data copying on iOS

build/copy_test_data_ios.gypi

Index: build/copy_test_data_ios.gypi
diff --git a/build/copy_test_data_ios.gypi b/build/copy_test_data_ios.gypi
index 56a222f9f84375b957ea6d170cf8e6c3ce3954a2..576a0f26ecbe474c56dec81b541bd57acb3da71f 100644
--- a/build/copy_test_data_ios.gypi
+++ b/build/copy_test_data_ios.gypi
@@ -34,7 +34,12 @@
 
 {
   'inputs': [
-    '<!@pymod_do_main(copy_test_data_ios --inputs <(test_data_files))',
+    # The |-o <(test_data_prefix)| is ignored; it is there to work around a
+    # caching bug in gyp (https://code.google.com/p/gyp/issues/detail?id=112).
+    # It caches command output when the string is the same, so if two copy
+    # steps have the same relative paths, there can be bogus cache hits that
+    # cause compile failures unless something varies.
+    '<!@pymod_do_main(copy_test_data_ios -o <(test_data_prefix) --inputs <(test_data_files))',
   ],
   'outputs': [
     '<!@pymod_do_main(copy_test_data_ios -o <(PRODUCT_DIR)/<(_target_name).app/<(test_data_prefix) --outputs <(test_data_files))',