Add a command-line flag to enable threaded GPU rasterization.

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 f623cb1d2abdab6fe464aa2d7543e4112150f5ae..e3cb405a68357be49fd66a1a8d1dce4f1b46df28 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -239,6 +239,8 @@ void RenderWidgetCompositor::Initialize() {
       compositor_deps_->IsGpuRasterizationForced();
   settings.gpu_rasterization_enabled =
       compositor_deps_->IsGpuRasterizationEnabled();
+  settings.threaded_gpu_rasterization_enabled =
+      compositor_deps_->IsThreadedGpuRasterizationEnabled();
   settings.can_use_lcd_text = compositor_deps_->IsLcdTextEnabled();
   settings.use_distance_field_text =
       compositor_deps_->IsDistanceFieldTextEnabled();