Add minimal support for isolating iOS targets.

Currently the iOS recipes don't use MB to generate isolates,
and so MB's isolate support for iOS is not surprisingly kinda broken.

This CL fixes things at least a bit, so that we can at least generate
isolates. Not sure what else we'll find that's broken.

Change-Id: Iac1c6d5ea0dcd1a386ac164550b224ed1ebf408b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542223
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Dirk Pranke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#647409}
diff --git a/tools/mb/mb.py b/tools/mb/mb.py
index a949d77..e436d03f 100755
--- a/tools/mb/mb.py
+++ b/tools/mb/mb.py
@@ -940,6 +940,7 @@
     """
 
     android = 'target_os="android"' in vals['gn_args']
+    ios = 'target_os="ios"' in vals['gn_args']
     fuchsia = 'target_os="fuchsia"' in vals['gn_args']
     win = self.platform == 'win32' or 'target_os="win"' in vals['gn_args']
     possible_runtime_deps_rpaths = {}
@@ -961,8 +962,8 @@
         rpaths = [
             target + '.runtime_deps',
             'obj/%s.stamp.runtime_deps' % label.replace(':', '/')]
-      elif fuchsia:
-        # Only emit a runtime deps file for the group() target on Fuchsia.
+      elif ios or fuchsia:
+        # iOS and Fuchsia targets end up as groups.
         label = isolate_map[target]['label']
         rpaths = ['obj/%s.stamp.runtime_deps' % label.replace(':', '/')]
       elif (isolate_map[target]['type'] == 'script' or