Check for an empty V8 context in memory measurement experiment

The timer that starts the memory measurement in the Finch experiment
can fire for a frame that was detached leading to a nullptr access.

Bug: 1105494
Change-Id: I37a2b067acb24303fc5da5a7b7d2855445f28847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2320672Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792253}

third_party/blink/renderer/core/timing/window_performance.cc

@@ -510,6 +510,10 @@ void WindowPerformance::MeasureMemoryExperimentTimerFired(TimerBase*) {
   v8::HandleScope handle_scope(isolate);
   v8::Local<v8::Context> context =
       ToV8Context(GetFrame(), DOMWrapperWorld::MainWorld());
+  if (context.IsEmpty()) {
+    // The frame has been detached in the meantime.
+    return;
+  }
   isolate->MeasureMemory(
       std::make_unique<MeasureMemoryDelegate>(isolate, context),
       v8::MeasureMemoryExecution::kDefault);