cc: Add experimental --num-raster-threads option.

cc/switches.cc

Index: cc/switches.cc
diff --git a/cc/switches.cc b/cc/switches.cc
index b7670b7860f5a1a9e69f40c227d5f40348f0a69e..7b501db3c30e04970dc06eb6eacd94d46ff4345f 100644
--- a/cc/switches.cc
+++ b/cc/switches.cc
@@ -25,6 +25,9 @@ const char kEnablePerTilePainting[]         = "enable-per-tile-painting";
 // compositor.
 const char kEnablePinchInCompositor[]       = "enable-pinch-in-compositor";
 
+// Number of worker threads used to rasterize content.
+const char kNumRasterThreads[] = "num-raster-threads";
+
 // Show rects in the HUD around layers whose properties have changed.
 const char kShowPropertyChangedRects[] = "show-property-changed-rects";