cppgc: Provide default implementation of Platform::GetTracingController

Bug: chromium:1056170
Change-Id: I53e0e45045282c1fd217af34ff31b7e6411624b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2552508
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71316}

include/cppgc/platform.h

@@ -116,9 +116,11 @@ class V8_EXPORT Platform {
   }
 
   /**
-   * Returns an instance of a TracingController. This must be non-nullptr.
+   * Returns an instance of a `TracingController`. This must be non-nullptr. The
+   * default implementation returns an empty `TracingController` that consumes
+   * trace data without effect.
    */
-  virtual TracingController* GetTracingController() = 0;
+  virtual TracingController* GetTracingController();
 };
 
 /**

src/heap/cppgc/platform.cc

@@ -4,11 +4,17 @@
 
 #include "include/cppgc/platform.h"
 
+#include "src/base/lazy-instance.h"
 #include "src/base/platform/platform.h"
 #include "src/heap/cppgc/gc-info-table.h"
 
 namespace cppgc {
 
+TracingController* Platform::GetTracingController() {
+  static v8::base::LeakyObject<TracingController> tracing_controller;
+  return tracing_controller.get();
+}
+
 void InitializeProcess(PageAllocator* page_allocator) {
   internal::GlobalGCInfoTable::Create(page_allocator);
 }