Force software rasterization on ThreadTimesCompositorCases benchmark.

compositor-perf team is using these benchmarks for tracking compositor
performance primarily in software raster path, so forcing software raster.

BUG=407927

Review URL: https://codereview.chromium.org/516623002

Cr-Commit-Position: refs/heads/master@{#292427}

tools/perf/benchmarks/silk_flags.py

@@ -6,6 +6,9 @@ def CustomizeBrowserOptionsForFastPath(options):
   """Enables flags needed for bleeding edge rendering fast paths."""
   options.AppendExtraBrowserArgs('--enable-bleeding-edge-rendering-fast-paths')
 
+def CustomizeBrowserOptionsForSoftwareRasterization(options):
+  """Enables flags needed for forced software rasterization."""
+  options.AppendExtraBrowserArgs('--disable-gpu-rasterization')
 
 def CustomizeBrowserOptionsForGpuRasterization(options):
   """Enables flags needed for forced GPU rasterization using Ganesh."""

tools/perf/benchmarks/thread_times.py

@@ -54,11 +54,13 @@ class ThreadTimesSimpleMobileSites(benchmark.Benchmark):
 
 class ThreadTimesCompositorCases(benchmark.Benchmark):
   """Measures timeline metrics while performing smoothness action on
-  tough compositor cases.
+  tough compositor cases, using software rasterization.
+
   http://www.chromium.org/developers/design-documents/rendering-benchmarks"""
   test = thread_times.ThreadTimes
   page_set = page_sets.ToughCompositorCasesPageSet
-
+  def CustomizeBrowserOptions(self, options):
+    silk_flags.CustomizeBrowserOptionsForSoftwareRasterization(options)
 
 @benchmark.Enabled('android')
 class ThreadTimesPolymer(benchmark.Benchmark):