Dynamically create stencil buffer when needed.

Review URL: https://codereview.chromium.org/938383004
diff --git a/gyp/gpu.gypi b/gyp/gpu.gypi
index 9803c42..e17cb7a 100644
--- a/gyp/gpu.gypi
+++ b/gyp/gpu.gypi
@@ -149,6 +149,7 @@
       '<(skia_src_path)/gpu/GrRectanizer_skyline.h',
       '<(skia_src_path)/gpu/GrRedBlackTree.h',
       '<(skia_src_path)/gpu/GrRenderTarget.cpp',
+      '<(skia_src_path)/gpu/GrRenderTargetPriv.h',
       '<(skia_src_path)/gpu/GrReducedClip.cpp',
       '<(skia_src_path)/gpu/GrReducedClip.h',
       '<(skia_src_path)/gpu/GrResourceCache.cpp',