Add a command-line flag to set GPU rasterization multisampling.

content/test/fake_compositor_dependencies.cc

Index: content/test/fake_compositor_dependencies.cc
diff --git a/content/test/fake_compositor_dependencies.cc b/content/test/fake_compositor_dependencies.cc
index bf24bab9be6ddcafd9bb1643f961066c38858dc5..baa5181bfe0b7f86581b26e444e520c877319f8a 100644
--- a/content/test/fake_compositor_dependencies.cc
+++ b/content/test/fake_compositor_dependencies.cc
@@ -25,6 +25,10 @@ bool FakeCompositorDependencies::IsGpuRasterizationEnabled() {
   return false;
 }
 
+int FakeCompositorDependencies::GpuRasterizationMSAASampleCount() {
+  return 0;
+}
+
 bool FakeCompositorDependencies::IsLcdTextEnabled() {
   return false;
 }