run Test-iOS-... bots with --threads 0.

Yet another attempt to get the iOS bots stable.

As GPU testers, the iOS bots are already mostly single threaded,
but farm a good chunk of work like image encoding out to threads.

This will take them all the way to really single threaded.

BUG=skia:6748

Change-Id: I222e3ca402eee9d191c8a413b29a65d19c13e88a
Reviewed-on: https://skia-review.googlesource.com/20687
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

infra/bots/recipes/test.expected/Test-iOS-Clang-iPadMini4-GPU-GX6450-arm-Release.json

@@ -725,6 +725,8 @@
       "dm",
       "--nocpu",
       "--randomProcessorTest",
+      "--threads",
+      "0",
       "--config",
       "8888",
       "srgb",

infra/bots/recipes/test.py

@@ -34,11 +34,15 @@ def dm_flags(bot):
   # 32-bit desktop bots tend to run out of memory, because they have relatively
   # far more cores than RAM (e.g. 32 cores, 3G RAM).  Hold them back a bit.
   if '-x86-' in bot and not 'NexusPlayer' in bot:
-    args.extend('--threads 4'.split(' '))
+    args.extend(['--threads', '4'])
 
   # Avoid issues with dynamically exceeding resource cache limits.
   if 'Test' in bot and 'DISCARDABLE' in bot:
-    args.extend('--threads 0'.split(' '))
+    args.extend(['--threads', '0'])
+
+  # See if staying on the main thread helps skia:6748.
+  if 'Test-iOS' in bot:
+    args.extend(['--threads', '0'])
 
   # These are the canonical configs that we would ideally run on all bots. We
   # may opt out or substitute some below for specific bots