Make skpaint->grpaint flow work for composing draws

gyp/gpu.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # Include this gypi to include all 'gpu' files
 # The parent gyp/gypi file must define
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi
@@ skipping 361 matching lines @@
       '<(skia_include_path)/gpu/SkGr.h',
       '<(skia_include_path)/gpu/SkGrPixelRef.h',
       '<(skia_include_path)/gpu/SkGrTexturePixelRef.h',
 
       '<(skia_include_path)/gpu/gl/SkGLContext.h',
 
       '<(skia_src_path)/gpu/SkGpuDevice.cpp',
       '<(skia_src_path)/gpu/SkGpuDevice.h',
       '<(skia_src_path)/gpu/SkGr.cpp',
       '<(skia_src_path)/gpu/SkGrPixelRef.cpp',
+      '<(skia_src_path)/gpu/SkGrPriv.h',
       '<(skia_src_path)/gpu/SkGrTexturePixelRef.cpp',
 
       '<(skia_src_path)/image/SkImage_Gpu.h',
       '<(skia_src_path)/image/SkImage_Gpu.cpp',
       '<(skia_src_path)/image/SkSurface_Gpu.h',
       '<(skia_src_path)/image/SkSurface_Gpu.cpp',
 
       '<(skia_src_path)/gpu/gl/SkGLContext.cpp'
     ],
     'skgpu_native_gl_sources': [
@@ skipping 56 matching lines @@
       '<(skia_src_path)/gpu/gl/debug/SkDebugGLContext.cpp',
       '<(skia_src_path)/gpu/gl/debug/SkDebugGLContext.h',
     ],
     'skgpu_null_gl_sources': [
       '<(skia_src_path)/gpu/gl/GrGLCreateNullInterface.cpp',
       '<(skia_src_path)/gpu/gl/SkNullGLContext.cpp',
       '<(skia_include_path)/gpu/gl/SkNullGLContext.h',
     ],
   },
 }