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

cc/resources/gpu_rasterizer.h

Index: cc/resources/gpu_rasterizer.h
diff --git a/cc/resources/gpu_rasterizer.h b/cc/resources/gpu_rasterizer.h
index cbc93853c192299283e507145dd492ceb1327d44..61243f9119e4d93f884d1b5bf6e0bbbc63e053d8 100644
--- a/cc/resources/gpu_rasterizer.h
+++ b/cc/resources/gpu_rasterizer.h
@@ -25,7 +25,8 @@ class CC_EXPORT GpuRasterizer : public Rasterizer {
   static scoped_ptr<GpuRasterizer> Create(ContextProvider* context_provider,
                                           ResourceProvider* resource_provider,
                                           bool use_distance_field_text,
-                                          bool tile_prepare_enabled);
+                                          bool tile_prepare_enabled,
+                                          int msaa_sample_count);
   PrepareTilesMode GetPrepareTilesMode() override;
   void RasterizeTiles(
       const TileVector& tiles,
@@ -36,7 +37,8 @@ class CC_EXPORT GpuRasterizer : public Rasterizer {
   GpuRasterizer(ContextProvider* context_provider,
                 ResourceProvider* resource_provider,
                 bool use_distance_filed_text,
-                bool tile_prepare_enabled);
+                bool tile_prepare_enabled,
+                int msaa_sample_count);
 
   using ScopedResourceWriteLocks =
       ScopedPtrVector<ResourceProvider::ScopedWriteLockGr>;
@@ -53,6 +55,7 @@ class CC_EXPORT GpuRasterizer : public Rasterizer {
 
   bool use_distance_field_text_;
   bool tile_prepare_enabled_;
+  int msaa_sample_count_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuRasterizer);
 };