Add gyp support for new GPU backend

Review URL: https://codereview.chromium.org/1287023006
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index ed4cdb6..715f08e 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -134,6 +134,7 @@
       'skia_osx_deployment_target%': '',
       'skia_pdf%': 1,
       'skia_profile_enabled%': 0,
+      'skia_vulkan%': 0,
       'skia_win_debuggers_path%': '',
       'skia_shared_lib%': 0,
       'skia_opencl%': 0,
@@ -207,6 +208,7 @@
     'skia_chrome_utils%': '<(skia_chrome_utils)',
     'skia_gdi%': '<(skia_gdi)',
     'skia_gpu%': '<(skia_gpu)',
+    'skia_vulkan%': '<(skia_vulkan)',
     'skia_win_exceptions%': 0,
     'skia_win_ltcg%': 1,
     'sknx_no_simd%': 0,