Composite fullscreen videos with composited descendants.

This fixes the referenced performance regression in which some such
videos stopped being composited.

Bug: 1108972

Change-Id: Ia77d33c1c50cf25427d825ae3969a120a8cde640
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335648Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794573}

third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater.cc

@@ -27,6 +27,7 @@
 #include "third_party/blink/renderer/core/paint/compositing/compositing_requirements_updater.h"
 
 #include "base/macros.h"
+#include "third_party/blink/renderer/core/html/media/html_media_element.h"
 #include "third_party/blink/renderer/core/layout/layout_embedded_content.h"
 #include "third_party/blink/renderer/core/layout/layout_view.h"
 #include "third_party/blink/renderer/core/paint/compositing/paint_layer_compositor.h"
@@ -183,6 +184,13 @@ static CompositingReasons SubtreeReasonsForCompositing(
     subtree_reasons |= CompositingReason::kIsolateCompositedDescendants;
   }
 
+  if (layer->GetLayoutObject().IsVideo() &&
+      To<HTMLMediaElement>(layer->GetLayoutObject().GetNode())
+          ->IsFullscreen()) {
+    subtree_reasons |=
+        CompositingReason::kFullscreenVideoWithCompositedDescendants;
+  }
+
   // A layer with preserve-3d or perspective only needs to be composited if
   // there are descendant layers that will be affected by the preserve-3d or
   // perspective.

third_party/blink/renderer/platform/graphics/compositing_reasons.cc

@@ -104,6 +104,9 @@ constexpr CompositingReasonStringMap kCompositingReasonsStringMap[] = {
     {CompositingReason::kIsolateCompositedDescendants,
      "isolateCompositedDescendants",
      "Should isolate descendants to apply a blend effect"},
+    {CompositingReason::kFullscreenVideoWithCompositedDescendants,
+     "fullscreenVideoWithCompositedDescendants",
+     "Is a fullscreen video element with composited descendants"},
     {CompositingReason::kRoot, "root", "Is the root layer"},
     {CompositingReason::kLayerForHorizontalScrollbar,
      "layerForHorizontalScrollbar",

third_party/blink/renderer/platform/graphics/compositing_reasons.h

@@ -64,6 +64,7 @@ using CompositingReasons = uint64_t;
   V(PerspectiveWith3DDescendants)                                             \
   V(Preserve3DWith3DDescendants)                                              \
   V(IsolateCompositedDescendants)                                             \
+  V(FullscreenVideoWithCompositedDescendants)                                 \
                                                                               \
   /* The root layer is a special case. It may be forced to be a layer, but it \
   also needs to be a layer if anything else in the subtree is composited. */  \