Use uint64_t for PlatformFence

VkFence is 64 bit even on 32-bit platforms.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2391113002

Review-Url: https://codereview.chromium.org/2391113002
diff --git a/tools/gpu/vk/VkTestContext.cpp b/tools/gpu/vk/VkTestContext.cpp
index c1a2f8a..a76a361 100644
--- a/tools/gpu/vk/VkTestContext.cpp
+++ b/tools/gpu/vk/VkTestContext.cpp
@@ -103,6 +103,8 @@
     typedef sk_gpu_test::FenceSync INHERITED;
 };
 
+GR_STATIC_ASSERT(sizeof(VkFence) <= sizeof(sk_gpu_test::PlatformFence));
+
 // TODO: Implement swap buffers and finish
 class VkTestContextImpl : public sk_gpu_test::VkTestContext {
 public: