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

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 5ae4367758474a7ee2b815b4a9cafd6a3e8d3d87..cefc069902e44121744b33e30c08e138ab940cf4 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -498,6 +498,14 @@ const char kForceGpuRasterization[]        = "force-gpu-rasterization";
 const char kGpuRasterizationMSAASampleCount[] =
     "gpu-rasterization-msaa-sample-count";
 
+// Enable threaded GPU rasterization.
+const char kEnableThreadedGpuRasterization[] =
+    "enable-threaded-gpu-rasterization";
+
+// Disable threaded GPU rasterization.  Overrides enable.
+const char kDisableThreadedGpuRasterization[] =
+    "disable-threaded-gpu-rasterization";
+
 // Force renderer accessibility to be on instead of enabling it on demand when
 // a screen reader is detected. The disable-renderer-accessibility switch
 // overrides this if present.