discardable_memory: Add trace events to inspect background purge.

Bug: 1108911
Change-Id: Id43fa27447b9a531e60de0390224d35f8f437dfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325921Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793693}

components/discardable_memory/client/client_discardable_shared_memory_manager.cc

@@ -295,6 +295,8 @@ size_t ClientDiscardableSharedMemoryManager::GetBytesAllocated() const {
 }
 
 void ClientDiscardableSharedMemoryManager::ReleaseFreeMemory() {
+  TRACE_EVENT0("blink",
+               "ClientDiscardableSharedMemoryManager::ReleaseFreeMemory()");
   base::AutoLock lock(lock_);
 
   size_t heap_size_prior_to_releasing_memory = heap_->GetSize();

content/renderer/render_thread_impl.cc

@@ -2144,6 +2144,7 @@ void RenderThreadImpl::OnRendererForegrounded() {
 }
 
 void RenderThreadImpl::ReleaseFreeMemory() {
+  TRACE_EVENT0("blink", "RenderThreadImpl::ReleaseFreeMemory()");
   base::allocator::ReleaseFreeMemory();
   discardable_memory_allocator_->ReleaseFreeMemory();
 

third_party/blink/renderer/platform/scheduler/main_thread/memory_purge_manager.cc

@@ -138,6 +138,7 @@ void MemoryPurgeManager::RequestMemoryPurgeWithDelay(base::TimeDelta delay) {
 }
 
 void MemoryPurgeManager::PerformMemoryPurge() {
+  TRACE_EVENT0("blink", "MemoryPurgeManager::PerformMemoryPurge()");
   DCHECK(CanPurge());
 
   base::MemoryPressureListener::NotifyMemoryPressure(