Reduce jank in card loading.

Previously, ManagedWebContents would inform callbacks that it was ready
for embedding after WebView creation. Now, we wait until after the
first load has stopped. This prevents embedding from happening until
all assets have been loaded.

Previously, large cards (e.g. "nearby restaurants") would occasionally
flicker as they were embedded before larger assets (e.g. map image)
were finished loading.

Bug: b:112498878
Change-Id: I0dce539800d5a18056e7d83a1078ccec70e7a25e
Reviewed-on: https://chromium-review.googlesource.com/1172067
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582643}

chrome/browser/ui/ash/assistant/web_contents_manager.cc

@@ -25,26 +25,29 @@
 
 // ManagedWebContents ----------------------------------------------------------
 
-class ManagedWebContents : public content::WebContentsDelegate {
+class ManagedWebContents : public content::WebContentsDelegate,
+                           public content::WebContentsObserver {
  public:
   ManagedWebContents(
       ash::mojom::ManagedWebContentsParamsPtr params,
-      ash::mojom::WebContentsManager::ManageWebContentsCallback callback) {
+      ash::mojom::WebContentsManager::ManageWebContentsCallback callback)
+      : callback_(std::move(callback)) {
     Profile* profile = chromeos::ProfileHelper::Get()->GetProfileByAccountId(
         params->account_id);
 
     if (!profile) {
       LOG(WARNING) << "Unable to retrieve profile for account_id.";
-      std::move(callback).Run(base::nullopt);
+      std::move(callback_).Run(base::nullopt);
       return;
     }
 
     InitWebContents(profile, std::move(params));
-    HandleWebContents(profile, std::move(callback));
+    HandleWebContents(profile);
   }
 
   ~ManagedWebContents() override {
     web_contents_->SetDelegate(nullptr);
+    Observe(nullptr);
 
     // When WebContents are rendered in the same process as ash, we need to
     // release the associated view registered in the
@@ -72,6 +75,14 @@ class ManagedWebContents : public content::WebContentsDelegate {
     return nullptr;
   }
 
+  // content::WebContentsObserver:
+  void DidStopLoading() override {
+    // After the first load has stopped, notify |callback_| that we're ready for
+    // embedding. We wait until load completion to reduce UI jank.
+    if (callback_)
+      std::move(callback_).Run(embed_token_);
+  }
+
  private:
   void InitWebContents(Profile* profile,
                        ash::mojom::ManagedWebContentsParamsPtr params) {
@@ -93,6 +104,7 @@ class ManagedWebContents : public content::WebContentsDelegate {
     views::WebContentsSetBackgroundColor::CreateForWebContentsWithColor(
         web_contents_.get(), SK_ColorTRANSPARENT);
 
+    Observe(web_contents_.get());
     web_contents_->SetDelegate(this);
 
     // Load the desired URL into the web contents.
@@ -118,9 +130,7 @@ class ManagedWebContents : public content::WebContentsDelegate {
                                                                max_size_dip);
   }
 
-  void HandleWebContents(
-      Profile* profile,
-      ash::mojom::WebContentsManager::ManageWebContentsCallback callback) {
+  void HandleWebContents(Profile* profile) {
     // When rendering WebContents in the same process as ash, we register the
     // associated view with the AnswerCardContentsRegistry's token-to-view map.
     // The token returned from the registry will uniquely identify the view.
@@ -132,14 +142,13 @@ class ManagedWebContents : public content::WebContentsDelegate {
 
       embed_token_ = app_list::AnswerCardContentsRegistry::Get()->Register(
           web_view_.get());
-
-      std::move(callback).Run(embed_token_.value());
     } else {
       // TODO(dmblack): Handle Mash case. https://crbug.com/854787.
-      std::move(callback).Run(base::nullopt);
     }
   }
 
+  ash::mojom::WebContentsManager::ManageWebContentsCallback callback_;
+
   std::unique_ptr<content::WebContents> web_contents_;
   std::unique_ptr<views::WebView> web_view_;
   base::Optional<base::UnguessableToken> embed_token_;