Add GN buildfile for gfx

skia/skia_gn_files.gypi

 # This file is read into the GN build.
 
 # Files are relative to third_party/skia.
 {
   # Shared defines for all builds.
   'skia_feature_defines': [
     'GR_GL_CUSTOM_SETUP_HEADER="GrGLConfig_chrome.h"',
     'GR_GL_IGNORE_ES3_MSAA=0',
     'SK_ATTR_DEPRECATED=SK_NOTHING_ARG1',
     'SK_DEFERRED_CANVAS_USES_FACTORIES=1',
@@ skipping 362 matching lines @@
     'src/core/SkPathHeap.h',
     'src/core/SkPathMeasure.cpp',
     'src/core/SkPathRef.cpp',
     'src/core/SkPicture.cpp',
     'src/core/SkPictureFlat.cpp',
     'src/core/SkPictureFlat.h',
     'src/core/SkPicturePlayback.cpp',
     'src/core/SkPicturePlayback.h',
     'src/core/SkPictureRecord.cpp',
     'src/core/SkPictureRecord.h',
+    'src/core/SkPictureShader.cpp',
+    'src/core/SkPictureShader.h',
     'src/core/SkPictureStateTree.cpp',
     'src/core/SkPictureStateTree.h',
     'src/core/SkPixelRef.cpp',
     'src/core/SkPoint.cpp',
     'src/core/SkProcSpriteBlitter.cpp',
     'src/core/SkPtrRecorder.cpp',
     'src/core/SkQuadClipper.cpp',
     'src/core/SkQuadClipper.h',
     'src/core/SkQuadTree.cpp',
     'src/core/SkQuadTree.h',
@@ skipping 478 matching lines @@
     'src/utils/win/SkDWriteGeometrySink.h',
     'src/utils/win/SkHRESULT.cpp',
     'src/utils/win/SkIStream.cpp',
     'src/utils/win/SkWGL_win.cpp',
 
     #testing
     'src/fonts/SkGScalerContext.cpp',
     'src/fonts/SkGScalerContext.h',
   ],
 }