Add a MotionMark smoothness benchmark.

tools/perf/benchmarks/smoothness.py

Index: tools/perf/benchmarks/smoothness.py
diff --git a/tools/perf/benchmarks/smoothness.py b/tools/perf/benchmarks/smoothness.py
index 733eb121c073789b7da524dc4c050f701fd5b780..f77a922127f2c61345ad021c2aab1fdc5479f30b 100644
--- a/tools/perf/benchmarks/smoothness.py
+++ b/tools/perf/benchmarks/smoothness.py
@@ -542,3 +542,15 @@ class SmoothnessToughWebGLAdCases(_Smoothness):
   @classmethod
   def ShouldDisable(cls, possible_browser):
     return cls.IsSvelte(possible_browser)  # http://crbug.com/574485
+
+class SmoothnessMotionmark(_Smoothness):
+  """Measures frame rate and a variety of other statistics on MotionMark cases.
+  """
+  page_set = page_sets.MotionMarkPageSet
+
+  def SetExtraBrowserOptions(self, options):
+    options.AppendExtraBrowserArgs('--enable-experimental-canvas-features')
+
+  @classmethod
+  def Name(cls):
+    return 'smoothness.motionmark'