Move Vulkan context to EmbedderTestContextVulkan (#56571)

In #29391 we extracted EmbedderTestContextVulkan, but didn't move the vulkan_context_ to that class. Since it's no longer used in the base class, we push it down to avoid the ifdef guards.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

shell/platform/embedder/tests/embedder_test_context.h

@@ -116,12 +116,6 @@ class EmbedderTestContext {
 
   using NextSceneCallback = std::function<void(sk_sp<SkImage> image)>;
 
-#ifdef SHELL_ENABLE_VULKAN
-  // The TestVulkanContext destructor must be called _after_ the compositor is
-  // freed.
-  fml::RefPtr<TestVulkanContext> vulkan_context_ = nullptr;
-#endif
-
 #ifdef SHELL_ENABLE_GL
   std::shared_ptr<TestEGLContext> egl_context_ = nullptr;
 #endif

shell/platform/embedder/tests/embedder_test_context_vulkan.h

@@ -38,6 +38,10 @@ class EmbedderTestContextVulkan : public EmbedderTestContext {
                                 const char* name);
 
  private:
+  // The TestVulkanContext destructor must be called _after_ the compositor is
+  // freed.
+  fml::RefPtr<TestVulkanContext> vulkan_context_ = nullptr;
+
   std::unique_ptr<TestVulkanSurface> surface_;
 
   SkISize surface_size_ = SkISize::MakeEmpty();