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

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 2b645e804fa43698b09f1915b0465e7250d2bf5a..ea71a9bfaa70ba2adc0b0c2ec4902e29ab1393b5 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -232,6 +232,8 @@ void RenderWidgetCompositor::Initialize() {
   settings.max_untiled_layer_size = gfx::Size(max_untiled_layer_width,
                                            max_untiled_layer_height);
 
+  settings.gpu_rasterization_msaa_sample_count =
+      compositor_deps_->GpuRasterizationMSAASampleCount();
   settings.impl_side_painting = compositor_deps_->IsImplSidePaintingEnabled();
   settings.gpu_rasterization_forced =
       compositor_deps_->IsGpuRasterizationForced();