scheduler: Always create a real scheduler in unit tests

content/test/DEPS

 include_rules = [
   # Allow inclusion of specific components that we depend on. We may only
   # depend on components which we share with the mojo html_viewer.
   "+components/scheduler/renderer",
+  "+components/scheduler/test",
 
   "+cc/blink",
   "+chromeos/audio", # For WebRTC tests.
   # Testing utilities can access anything in content/
   "+content",
   "+device/bluetooth", # For WebBluetooth tests
   # For loading V8's initial snapshot from external files.
   "+gin/v8_initializer.h",
   "+media/audio", # For AudioParameters in WebRTC tests.
   "+media/base",  # For ChannelLayout in WebRTC tests.
   "+ui/base/resource/data_pack.h",
   "+ui/base/resource/resource_bundle.h",
   "!v8/include/v8.h",
 ]
 
 specific_include_rules = {
   "layouttest_support.cc": [
     "+components/test_runner",
   ],
 }