Make trigger_multiple_dimensions.py work for iOS

iOS swarming arguments doesn't contain isolate args, which are separated
by '--' from the swarming trigger arguments.  Remove the '--' assert in
the script which blocks iOS tasks to use the custom swarming trigger logic.
Other platforms:
<args to swarming.py trigger> -- <args to bot running isolate>

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I49bb5a1c0abf78a5aa775d139f9dc60c60adeb09
Reviewed-on: https://chromium-review.googlesource.com/953251Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Menglu Huang <huangml@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541631}

testing/trigger_scripts/base_test_triggerer.py

@@ -74,12 +74,6 @@ class BaseTestTriggerer(object):
     list, to either affect the trigger command, or what the bot runs.
 
     """
-
-
-
-    assert '--' in all_args, (
-        'Malformed trigger command; -- argument expected but not found')
-    dash_ind = all_args.index('--')
     bot_args = ['--dump-json', temp_file]
     if total_shards > 1:
       bot_args.append('--env')
@@ -92,8 +86,12 @@ class BaseTestTriggerer(object):
       bot_args.append('--dimension')
       bot_args.append(key)
       bot_args.append(val)
-    return self.append_additional_args(
-        all_args[:dash_ind] + bot_args + all_args[dash_ind:])
+    if '--' in all_args:
+      dash_ind = all_args.index('--')
+      additional_args = all_args[:dash_ind] + bot_args + all_args[dash_ind:]
+    else:
+      additional_args = all_args + bot_args
+    return self.append_additional_args(additional_args)
 
   def append_additional_args(self, args):
     """ Gives subclasses ability to append additional args if necessary