[fuchsia] Disable memory mitigations for visible LayerTreeHostImpls.

Attempting to unlock/release memory used by visible LayerTreeHostImpls
on receipt of CRITICAL memory pressure notifications causes flickering
when the content needs to be re-composited.

Disable these mitigations for visible instances, and re-enable memory
pressure handling in WebEngine renderer processes by default.

Bug: 1189208
Bug: 1175858, 1173685
Bug: b/181245064
Change-Id: Iee9160b312299d53e665d03991fe3e29019786ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773224
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864726}

cc/trees/layer_tree_host_impl.cc

@@ -3308,6 +3308,13 @@ void LayerTreeHostImpl::OnMemoryPressure(
   if (level != base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL)
     return;
 
+    // TODO(crbug.com/1189208): Unlocking decoded-image-tracker images causes
+    // flickering in visible trees if Out-Of-Process rasterization is enabled.
+#if defined(OS_FUCHSIA)
+  if (use_oop_rasterization() && visible())
+    return;
+#endif  // defined(OS_FUCHSIA)
+
   ReleaseTileResources();
   active_tree_->OnPurgeMemory();
   if (pending_tree_)
@@ -3322,6 +3329,7 @@ void LayerTreeHostImpl::OnMemoryPressure(
   }
   if (resource_pool_)
     resource_pool_->OnMemoryPressure(level);
+
   tile_manager_.decoded_image_tracker().UnlockAllImages();
 }
 

fuchsia/engine/features.h

@@ -10,7 +10,7 @@
 namespace features {
 
 constexpr base::Feature kHandleMemoryPressureInRenderer{
-    "HandleMemoryPressureInRenderer", base::FEATURE_DISABLED_BY_DEFAULT};
+    "HandleMemoryPressureInRenderer", base::FEATURE_ENABLED_BY_DEFAULT};
 
 }  // namespace features