Add extra content_shell per worker for virtual tests, except on Android.

Tools/Scripts/webkitpy/layout_tests/port/base.py

Index: Tools/Scripts/webkitpy/layout_tests/port/base.py
diff --git a/Tools/Scripts/webkitpy/layout_tests/port/base.py b/Tools/Scripts/webkitpy/layout_tests/port/base.py
index 493cecf216bc675adc89a47bfc20766652170047..7c7541875a78a09037c5a297c8b36e96efe1b567 100644
--- a/Tools/Scripts/webkitpy/layout_tests/port/base.py
+++ b/Tools/Scripts/webkitpy/layout_tests/port/base.py
@@ -269,9 +269,13 @@ class Port(object):
         return None
 
     def default_child_processes(self):
-        """Return the number of drivers to use for this port."""
+        """Return the number of child processes to use for this port."""
         return self._executive.cpu_count()
 
+    def max_drivers_per_process(self):
+        """The maximum number of drivers a child process can use for this port."""
+        return 2
+
     def default_max_locked_shards(self):
         """Return the number of "locked" shards to run in parallel (like the http tests)."""
         max_locked_shards = int(self.default_child_processes()) / 4