Add gyp define to enable vulkan debug layers

gyp/common.gypi

 # Copyright 2011 The Android Open Source Project
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # This file is automatically included by gyp_skia when building any target.
 
 {
   'includes': [
     'common_variables.gypi',
@@ skipping 52 matching lines @@
       }],
       [ 'skia_vulkan', {
         'defines': [
           'SK_VULKAN',
         ],
         'direct_dependent_settings': {
           'defines': [
             'SK_VULKAN',
           ],
         },
+        'conditions': [
+          [ 'skia_vulkan_debug_layers', {
+            'configurations': {
+              'Debug':   { 'defines': [ 'SK_ENABLE_VK_LAYERS' ] },
+            },
+          }],
+        ],
       }],
       [ 'skia_command_buffer', {
         'defines': [
           'SK_COMMAND_BUFFER',
         ],
         'direct_dependent_settings': {
           'defines': [
             'SK_COMMAND_BUFFER',
           ],
         },
@@ skipping 32 matching lines @@
       [ 'skia_experimental_shadowing',
         {
           'defines': [
             'SK_EXPERIMENTAL_SHADOWING',
           ],
         },
       ],
     ],
   }, # end 'target_defaults'
 }