Update skia gn file to prep vulkan allocator changes.

Change-Id: Ib27e9405cc4c02cfdf83a6c2222f284ac57cad75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166490
Commit-Queue: Greg Daniel <egdaniel@google.com>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Auto-Submit: Greg Daniel <egdaniel@google.com>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762837}

skia/BUILD.gn

@@ -83,6 +83,7 @@ config("skia_config") {
     defines += [
       "SK_VULKAN_HEADER=\"../../skia/config/SkVulkanConfig.h\"",
       "SK_VULKAN=1",
+      "SK_USE_VMA",
     ]
   }
   if (skia_use_dawn) {