mac: Make pnacl and nacl irt optional in official build files list.

Chrome/mac/arm builds with nacl disabled.

Bug: 1117244
Change-Id: I1f44b9452b2d6692684c918ca200b3530a3a8bde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416742
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808025}

chrome/tools/build/mac/FILES.cfg

@@ -48,6 +48,7 @@ FILES = [
     'filename': 'pnacl',
     'buildtype': ['dev', 'official'],
     'archive': 'pnacl.zip',
+    'optional': ['dev', 'official'],  # Not present in arm builds.
   },
   # Browser tests:
   {
@@ -72,6 +73,7 @@ FILES = [
     'filename': 'nacl_irt_x86_64.nexe.debug',
     'buildtype': ['official'],
     'archive': 'chrome-mac-nacl-irt-syms.zip',
+    'optional': ['official'],  # Not present in arm builds.
   },
   # Content shell files:
   {