Two VR bisect fixes

Fixes two things in the VR auto bisect script:
1. Sets an empty list as the default for the checkout override arg, as
   otherwise it was trying to iterate over None when not set.
2. Ensure that the VR APKs are downloaded during syncing.

Change-Id: I2191accdc23fd858b0588c7772785ce4a95edc6a
Reviewed-on: https://chromium-review.googlesource.com/965115Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543534}

chrome/test/vr/auto_bisect.py

@@ -74,7 +74,8 @@ def ParseArgsAndAssertValid():
           'received %d' % len(split_arg))
     return {split_arg[0]: split_arg[1]}
   parser.add_argument('--checkout-override', action='append',
-                      type=comma_separated, dest='checkout_overrides',
+                      type=comma_separated, default=[],
+                      dest='checkout_overrides',
                       help='A comma-separated path/revision key/value pair. '
                            'Each git checkout at the specified path will be '
                            'synced to the specified revision after the normal '
@@ -396,6 +397,8 @@ def BisectRegression(args, unknown_args):
     unknown_args: The unknown args parsed by the argument parser
   """
   with TempDir() as output_dir:
+    # Ensure that we also sync any APKs we use
+    os.environ['DOWNLOAD_VR_TEST_APKS'] = '1'
     try:
       if args.good_value == None:
         args.good_value = GetValueAtRevision(args, unknown_args,