Fixed compile- and link-time errors with the "enable_vulkan" GN-argument

gpu/vulkan/BUILD.gn

 # Copyright 2016 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("//build/config/ui.gni")
 import("//testing/test.gni")
 
 config("vulkan_config") {
   defines = [ "ENABLE_VULKAN" ]
 }
 
 if (enable_vulkan) {
+  vulkan_lib_dir = getenv("VULKAN_SDK") + "/lib"
   component("vulkan") {
     output_name = "vulkan_wrapper"
 
     if (is_linux) {
       assert(use_x11, "Vulkan only support x11 at this point.")
       sources = [
         "vulkan_command_buffer.cc",
         "vulkan_command_buffer.h",
         "vulkan_command_pool.cc",
         "vulkan_command_pool.h",
@@ skipping 26 matching lines @@
       configs += [ "//build/config:precompiled_headers" ]
       defines = [ "VULKAN_IMPLEMENTATION" ]
 
       all_dependent_configs = [
         ":vulkan_config",
         "//third_party/vulkan:vulkan_headers",
       ]
       libs = [ "vulkan" ]
 
       if (current_cpu == "x64") {
-        lib_dirs = [ "/usr/lib/x86_64-linux-gnu" ]
+        lib_dirs = [
+          "/usr/lib/x86_64-linux-gnu",
+          vulkan_lib_dir,
+        ]
       } else {
         assert(false, "Unsupported vulkan target: " + current_cpu)
       }
 
       deps = [
         "//base",
         "//third_party/shaderc:libshaderc",
         "//ui/base",
         "//ui/gfx",
       ]
@@ skipping 10 matching lines @@
       "tests/basic_vulkan_test.cc",
       "tests/basic_vulkan_test.h",
       "tests/native_window.h",
       "tests/shader_module_unittest.cc",
       "tests/vulkan_test.cc",
       "tests/vulkan_tests_main.cc",
     ]
 
     include_dirs = [ "/usr/include" ]
     if (current_cpu == "x64") {
-      lib_dirs = [ "/usr/lib/x86_64-linux-gnu" ]
+      lib_dirs = [
+        "/usr/lib/x86_64-linux-gnu",
+        vulkan_lib_dir,
+      ]
     } else {
       assert(false, "Unsupported vulkan target: " + current_cpu)
     }
 
     deps = [
       ":vulkan",
       "//base/test:test_support",
       "//testing/gmock",
       "//testing/gtest",
     ]
 
     if (use_x11) {
       sources += [ "tests/native_window_x11.cc" ]
       deps += [ "//ui/gfx/x" ]
       configs += [ "//build/config/linux:x11" ]
     }
   }
 }