[Paint Preview] Change cleanup order and ensure objects exist

When under memory pressure, we need to make sure all objects exist and
that the callback is the last thing resolved. Otherwise `this` might be
gone.

Bug: 1148268
Change-Id: I0bc7c5ec29b4189f19b7936b20fc7cd6b8489d5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533900
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Auto-Submit: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826883}

components/paint_preview/player/player_compositor_delegate.cc

@@ -255,13 +255,17 @@ void PlayerCompositorDelegate::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
   if (memory_pressure_level ==
       base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL) {
+    if (paint_preview_compositor_client_)
+      paint_preview_compositor_client_.reset();
+
+    if (paint_preview_compositor_service_)
+      paint_preview_compositor_service_.reset();
+
     if (compositor_error_) {
       std::move(compositor_error_)
           .Run(static_cast<int>(
               CompositorStatus::STOPPED_DUE_TO_MEMORY_PRESSURE));
     }
-    paint_preview_compositor_client_.reset();
-    paint_preview_compositor_service_.reset();
   }
 }