MacViews: Fix DCHECK in CaptivePortalBrowserTest.GoBack

Allow BrowserCompositorMac to transition between different parent UI
layers, if needed.

Bug: 853302
Change-Id: I85596a1fe42c490d770f023c68d9a4093f050dcc
Reviewed-on: https://chromium-review.googlesource.com/1107047Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568716}

content/browser/renderer_host/browser_compositor_view_mac.h

@@ -196,7 +196,8 @@ class CONTENT_EXPORT BrowserCompositorMac : public DelegatedFrameHostClient,
 
   // Weak pointer to the layer supplied and reset via SetParentUiLayer. |this|
   // is an observer of |parent_ui_layer_|, to ensure that |parent_ui_layer_|
-  // always be valid when non-null.
+  // always be valid when non-null. The UpdateState function will re-parent
+  // |root_layer_| to be under |parent_ui_layer_|, if needed.
   ui::Layer* parent_ui_layer_ = nullptr;
   bool render_widget_host_is_hidden_ = true;
   bool ns_view_attached_to_window_ = false;

content/browser/renderer_host/browser_compositor_view_mac.mm

@@ -407,9 +407,12 @@ void BrowserCompositorMac::TransitionToState(State new_state) {
   // Note that the state enum values represent the other through which
   // transitions must be done (see comments in State definition).
 
-  // Transition UseParentLayerCompositor -> HasNoCompositor.
+  // Transition UseParentLayerCompositor -> HasNoCompositor. Note that this
+  // transition will be made if we are already in UseParentLayerCompositor, but
+  // with a different parent layer.
   if (state_ == UseParentLayerCompositor &&
-      new_state < UseParentLayerCompositor) {
+      (new_state < UseParentLayerCompositor ||
+       parent_ui_layer_ != root_layer_->parent())) {
     DCHECK(root_layer_->parent());
     root_layer_->parent()->RemoveObserver(this);
     root_layer_->parent()->Remove(root_layer_.get());
@@ -582,16 +585,16 @@ bool BrowserCompositorMac::ShouldContinueToPauseForFrame() const {
 }
 
 void BrowserCompositorMac::SetParentUiLayer(ui::Layer* new_parent_ui_layer) {
-  if (new_parent_ui_layer) {
+  if (new_parent_ui_layer)
     DCHECK(new_parent_ui_layer->GetCompositor());
-    DCHECK(!parent_ui_layer_);
-    parent_ui_layer_ = new_parent_ui_layer;
-  } else if (parent_ui_layer_) {
-    DCHECK(root_layer_->parent());
-    DCHECK_EQ(root_layer_->parent(), parent_ui_layer_);
-    parent_ui_layer_ = nullptr;
-  }
+
+  // Set |parent_ui_layer_| to the new value, which potentially not match the
+  // value of |root_layer_->parent()|. The call to UpdateState will re-parent
+  // |root_layer_|.
+  DCHECK_EQ(root_layer_->parent(), parent_ui_layer_);
+  parent_ui_layer_ = new_parent_ui_layer;
   UpdateState();
+  DCHECK_EQ(root_layer_->parent(), parent_ui_layer_);
 }
 
 bool BrowserCompositorMac::ForceNewSurfaceForTesting() {