Work around Xcode 9 debug actool message.

Xcode 9's beta introduced a CoreUI(DEBUG) message followed by a detail message
that can be ignored.  Presumably this will be removed once Xcode 9 GMs.

BUG=730054

Review-Url: https://codereview.chromium.org/2928623002
Cr-Original-Commit-Position: refs/heads/master@{#477535}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 44098da69108f7a5525edbfb307cf73c9902de66

toolchain/mac/compile_xcassets.py

@@ -62,6 +62,12 @@ def CompileXCAssets(
       continue
     if line == os.path.abspath(output):
       continue
+    # crbug.com/730054 Xcode 9's beta introduced a CoreUI(DEBUG) message and
+    # IBMessageChannelErrorDomain message that can be ignored.
+    if line.startswith('CoreUI(DEBUG)'):
+      continue
+    if 'Error Domain=IBMessageChannelErrorDomain Code=4' in line:
+      continue
     sys.stderr.write(stdout)
     sys.exit(1)