Make Shell::NotifyLowMemoryWarning trace

Author: dnfield

shell/common/shell.cc

@@ -430,18 +430,23 @@ Shell::~Shell() {
   platform_latch.Wait();
 }
 
-void Shell::NotifyLowMemoryWarning() const {
+void Shell::NotifyLowMemoryWarning() {
+  TRACE_EVENT_ASYNC_BEGIN0("flutter", "Shell::NotifyLowMemoryWarning",
+                           notify_low_memory_trace_id_);
   // This does not require a current isolate but does require a running VM.
   // Since a valid shell will not be returned to the embedder without a valid
   // DartVMRef, we can be certain that this is a safe spot to assume a VM is
   // running.
   ::Dart_NotifyLowMemory();
 
   task_runners_.GetRasterTaskRunner()->PostTask(
-      [rasterizer = rasterizer_->GetWeakPtr()]() {
+      [rasterizer = rasterizer_->GetWeakPtr(),
+       trace_id = notify_low_memory_trace_id_++]() {
         if (rasterizer) {
           rasterizer->NotifyLowMemoryWarning();
         }
+        TRACE_EVENT_ASYNC_END0("flutter", "Shell::NotifyLowMemoryWarning",
+                               trace_id);
       });
   // The IO Manager uses resource cache limits of 0, so it is not necessary
   // to purge them.

shell/common/shell.h

@@ -284,7 +284,7 @@ class Shell final : public PlatformView::Delegate,
   /// @brief      Used by embedders to notify that there is a low memory
   ///             warning. The shell will attempt to purge caches. Current, only
   ///             the rasterizer cache is purged.
-  void NotifyLowMemoryWarning() const;
+  void NotifyLowMemoryWarning();
 
   //----------------------------------------------------------------------------
   /// @brief      Used by embedders to check if all shell subcomponents are
@@ -397,6 +397,7 @@ class Shell final : public PlatformView::Delegate,
   std::atomic<bool> waiting_for_first_frame_ = true;
   std::mutex waiting_for_first_frame_mutex_;
   std::condition_variable waiting_for_first_frame_condition_;
+  size_t notify_low_memory_trace_id_ = 0;
 
   // Written in the UI thread and read from the raster thread. Hence make it
   // atomic.