Introduce Isolate::ClearCachesForTesting API method

This CL adds a new method intended for tests or lab settings to
cleanup V8 caches. The synchronous nature of the method greatly reduces
flakiness of blink leak detection in many cases.

Bug: chromium:1217831
Change-Id: I107eddc8b88d91aa7e69430ecfc135fe39538a5c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2948666
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75041}

include/v8.h

@@ -8719,6 +8719,13 @@ class V8_EXPORT Isolate {
    */
   void MemoryPressureNotification(MemoryPressureLevel level);
 
+  /**
+   * Drop non-essential caches. Should only be called from testing code.
+   * The method can potentially block for a long time and does not necessarily
+   * trigger GC.
+   */
+  void ClearCachesForTesting();
+
   /**
    * Methods below this point require holding a lock (using Locker) in
    * a multi-threaded environment.

src/api/api.cc

@@ -8906,6 +8906,12 @@ void Isolate::MemoryPressureNotification(MemoryPressureLevel level) {
   isolate->heap()->MemoryPressureNotification(level, on_isolate_thread);
 }
 
+void Isolate::ClearCachesForTesting() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  isolate->AbortConcurrentOptimization(i::BlockingBehavior::kBlock);
+  isolate->ClearSerializerData();
+}
+
 void Isolate::EnableMemorySavingsMode() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   isolate->EnableMemorySavingsMode();