Bug 1547892 - Load video element in VideoDocument slightly later during load. r=bzbarsky

Currently when we create the video inside a VideoDocument, the PresShell isn't
created yet. This means the video element can't access information about the
compositor, which means it doesn't know whether it can create a hardware
accelerated video decoder, and so we end up falling back to using a software
decoder.

So this patch moves the creation of the video element to slightly later in the
load of a VideoDocument, so that the PresShell is available when we create the
VideoDocument's video element. This means VideoDocuments's video decoder can be
hardware accelerated

Differential Revision: https://phabricator.services.mozilla.com/D30614

--HG--
extra : moz-landing-system : lando

dom/html/MediaDocument.h

@@ -53,7 +53,7 @@ class MediaDocument : public nsHTMLDocument {
   virtual nsresult CreateSyntheticDocument();
 
   friend class MediaDocumentStreamListener;
-  nsresult StartLayout();
+  virtual nsresult StartLayout();
 
   void GetFileName(nsAString& aResult, nsIChannel* aChannel);
 

dom/html/PluginDocument.cpp

@@ -47,8 +47,6 @@ class PluginDocument final : public MediaDocument, public nsIPluginDocument {
   const nsCString& GetType() const { return mMimeType; }
   Element* GetPluginContent() { return mPluginContent; }
 
-  void StartLayout() { MediaDocument::StartLayout(); }
-
   virtual void Destroy() override {
     if (mStreamListener) {
       mStreamListener->DropDocumentRef();

dom/html/VideoDocument.cpp

@@ -38,12 +38,13 @@ class VideoDocument final : public MediaDocument {
     MediaDocument::Destroy();
   }
 
+  nsresult StartLayout() override;
+
  protected:
+  nsresult CreateVideoElement();
   // Sets document <title> to reflect the file name and description.
   void UpdateTitle(nsIChannel* aChannel);
 
-  nsresult CreateSyntheticVideoDocument();
-
   RefPtr<MediaDocumentStreamListener> mStreamListener;
 };
 
@@ -62,6 +63,22 @@ nsresult VideoDocument::StartDocumentLoad(const char* aCommand,
   return rv;
 }
 
+nsresult VideoDocument::StartLayout() {
+  // Create video element, and begin loading the media resource. Note we
+  // delay creating the video element until now (we're called from
+  // MediaDocumentStreamListener::OnStartRequest) as the PresShell is likely
+  // to have been created by now, so the MediaDecoder will be able to tell
+  // what kind of compositor we have, so the video element knows whether
+  // it can create a hardware accelerated video decoder or not.
+  nsresult rv = CreateVideoElement();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = MediaDocument::StartLayout();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
 void VideoDocument::SetScriptGlobalObject(
     nsIScriptGlobalObject* aScriptGlobalObject) {
   // Set the script global object on the superclass before doing
@@ -69,11 +86,7 @@ void VideoDocument::SetScriptGlobalObject(
   MediaDocument::SetScriptGlobalObject(aScriptGlobalObject);
 
   if (aScriptGlobalObject && !InitialSetupHasBeenDone()) {
-    // Create synthetic document
-#ifdef DEBUG
-    nsresult rv =
-#endif
-        CreateSyntheticVideoDocument();
+    DebugOnly<nsresult> rv = CreateSyntheticDocument();
     NS_ASSERTION(NS_SUCCEEDED(rv), "failed to create synthetic video document");
 
     if (!nsContentUtils::IsChildOfSameType(this)) {
@@ -88,11 +101,7 @@ void VideoDocument::SetScriptGlobalObject(
   }
 }
 
-nsresult VideoDocument::CreateSyntheticVideoDocument() {
-  // make our generic document
-  nsresult rv = MediaDocument::CreateSyntheticDocument();
-  NS_ENSURE_SUCCESS(rv, rv);
-
+nsresult VideoDocument::CreateVideoElement() {
   Element* body = GetBodyElement();
   if (!body) {
     NS_WARNING("no body on video document!");