Add gyp define to enable vulkan debug layers

infra/bots/recipes/swarm_trigger.expected/Build-Win-MSVC-x86_64-Release-Vulkan.json

 [
   {
     "cmd": [
       "git",
       "rev-parse",
       "HEAD"
     ],
     "cwd": "[ROOT]/skia",
     "name": "git rev-parse",
     "stdout": "/path/to/tmp/"
@@ skipping 228 matching lines @@
       "@@@STEP_LOG_LINE@json.output@    \"role\": \"Build\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"target_arch\": \"x86_64\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"configuration\": \"Release_x64\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"dm_flags\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    \"--dummy-flags\"@@@",
       "@@@STEP_LOG_LINE@json.output@  ], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"do_perf_steps\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"do_test_steps\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"env\": {@@@",
-      "@@@STEP_LOG_LINE@json.output@    \"GYP_DEFINES\": \"qt_sdk=C:/Qt/4.8.5/ skia_arch_type=x86_64 skia_vulkan=1 skia_warnings_as_errors=1 skia_win_debuggers_path=c:/DbgHelp skia_win_ltcg=0\"@@@",
+      "@@@STEP_LOG_LINE@json.output@    \"GYP_DEFINES\": \"qt_sdk=C:/Qt/4.8.5/ skia_arch_type=x86_64 skia_vulkan=1 skia_vulkan_debug_layers=0 skia_warnings_as_errors=1 skia_win_debuggers_path=c:/DbgHelp skia_win_ltcg=0\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"nanobench_flags\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    \"--dummy-flags\"@@@",
       "@@@STEP_LOG_LINE@json.output@  ], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"upload_dm_results\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"upload_perf_results\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
@@ skipping 217 matching lines @@
       "@@@STEP_LINK@shard #0 isolated out@blah@@@",
       "@@@STEP_LINK@view steps on Milo@https://luci-milo.appspot.com/swarming/task/148aa78d7aa0000@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]