Update Skia GN build to use the new .gni file.

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
 # GN build files are placed when they can not be placed directly
 # in the source tree, e.g. for third party source trees.
@@ skipping 273 matching lines @@
   "//third_party/angle/src/tests/BUILD.gn",
   "//third_party/boringssl/BUILD.gn",
   "//third_party/catapult/tracing/BUILD.gn",
   "//third_party/cld_2/BUILD.gn",
   "//third_party/cython/rules.gni",
   "//third_party/google_input_tools/inputview.gni",
   "//third_party/harfbuzz-ng/BUILD.gn",
   "//third_party/libaddressinput/BUILD.gn",
   "//third_party/opus/BUILD.gn",
   "//third_party/openh264/BUILD.gn",
+  "//third_party/skia/gn/shared_sources.gni",
   "//third_party/WebKit/Source/bindings/bindings.gni",
   "//third_party/WebKit/Source/bindings/scripts/scripts.gni",
   "//third_party/WebKit/Source/config.gni",
   "//third_party/WebKit/Source/core/core.gni",
   "//third_party/WebKit/Source/modules/modules.gni",
   "//third_party/WebKit/Source/web/BUILD.gn",
   "//tools/gn/BUILD.gn",
   "//tools/gn/build_settings.h",
   "//tools/gn/command_help.cc",
   "//tools/gn/docs/language.md",
   "//tools/gn/format_test_data/053.gn",
   "//tools/gn/format_test_data/053.golden",
   "//tools/gn/format_test_data/055.gn",
   "//tools/gn/format_test_data/055.golden",
   "//tools/gn/function_exec_script.cc",
   "//tools/gn/gn.gyp",
   "//tools/gn/input_conversion.cc",
   "//tools/gn/misc/emacs/gn.el",
   "//tools/gn/misc/vim/syntax/gn.vim",
   "//tools/gn/setup.cc",
   "//ui/accessibility/BUILD.gn",
   "//ui/gfx/BUILD.gn",
   "//v8/test/cctest/BUILD.gn",
   "//v8/test/unittests/BUILD.gn",
 ]