cc: Make PaintCanvas abstract

cc/paint/BUILD.gn

 # Copyright 2017 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//cc/cc.gni")
 
 cc_component("paint") {
   output_name = "cc_paint"
   sources = [
     "paint_canvas.cc",
     "paint_canvas.h",
     "paint_export.h",
     "paint_flags.h",
     "paint_record.h",
     "paint_recorder.cc",
     "paint_recorder.h",
     "paint_shader.h",
     "paint_surface.cc",
     "paint_surface.h",
+    "skia_paint_canvas.cc",
+    "skia_paint_canvas.h",
   ]
 
   defines = [ "CC_PAINT_IMPLEMENTATION=1" ]
 
   # cc/paint is intended to be a separate component from cc that can be
   # included in Blink.  This component should never publicly include
   # anything that Blink core wouldn't include (e.g. base).
   public_deps = [
     "//skia",
   ]
 
   deps = [
     "//base",
   ]
 }