MSE: Rename and clarify the MediaSource attachment methods

AttachToElement() name was unclear: while it technically attaches the
MediaSource to the Element, the underlying MediaSource cannot be used
for appending media, etc. by the app until it is opened with a call to
SetWebMediaSourceAndOpen().

This change renames these two methods, respectively, to
{Start,Complete}AttachingToMediaElement(), along with some comment
updates. Actual function remains unchanged.

This clarification CL is expected to improve readability and review,
especially of the in-progress MSE-in-Workers CLs.

BUG=878133

Change-Id: Ic726cbee71d92e82a5a8c7d3be7d8d9185d1fb76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079384Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745588}

third_party/blink/renderer/core/html/media/html_media_element.cc

@@ -1192,7 +1192,7 @@ void HTMLMediaElement::LoadResource(const WebMediaPlayerSource& source,
 
   media_source_ = MediaSource::Lookup(url.GetString());
   if (media_source_) {
-    if (media_source_->AttachToElement(this)) {
+    if (media_source_->StartAttachingToMediaElement(this)) {
       // If the associated feature is enabled, auto-revoke the MediaSource
       // object URL that was used for attachment on successful (start of)
       // attachment. This can help reduce memory bloat later if the app does not
@@ -3992,7 +3992,8 @@ void HTMLMediaElement::SetCcLayer(cc::Layer* cc_layer) {
 
 void HTMLMediaElement::MediaSourceOpened(WebMediaSource* web_media_source) {
   SetShouldDelayLoadEvent(false);
-  media_source_->SetWebMediaSourceAndOpen(base::WrapUnique(web_media_source));
+  media_source_->CompleteAttachingToMediaElement(
+      base::WrapUnique(web_media_source));
 }
 
 bool HTMLMediaElement::IsInteractiveContent() const {

third_party/blink/renderer/core/html/media/media_source.h

@@ -54,16 +54,24 @@ class CORE_EXPORT MediaSource : public URLRegistrable,
                      : nullptr;
   }
 
-  // Called when an HTMLMediaElement is attempting to attach to this object,
-  // and helps enforce attachment to at most one element at a time.
-  // If already attached, returns false. Otherwise, must be in
-  // 'closed' state, and returns true to indicate attachment success.
+  // These two methods are called in sequence when an HTMLMediaElement is
+  // attempting to attach to this object.  The WebMediaSource is not available
+  // to the element initially, so between the two calls, the attachment could be
+  // considered partially setup.
+  // If already attached, StartAttachingToMediaElement() returns false.
+  // Otherwise, must be in 'closed' state, and returns true to indicate
+  // attachment success.
+  // CompleteAttachingToMediaElement() provides the MediaSource with the
+  // underlying WebMediaSource, enabling parsing of media provided by the
+  // application for playback, for example.
   // Reattachment allowed by first calling close() (even if already in
   // 'closed').
   // Once attached, the source uses the element to synchronously service some
   // API operations like duration change that may need to initiate seek.
-  virtual bool AttachToElement(HTMLMediaElement*) = 0;
-  virtual void SetWebMediaSourceAndOpen(std::unique_ptr<WebMediaSource>) = 0;
+  virtual bool StartAttachingToMediaElement(HTMLMediaElement*) = 0;
+  virtual void CompleteAttachingToMediaElement(
+      std::unique_ptr<WebMediaSource>) = 0;
+
   virtual void Close() = 0;
   virtual bool IsClosed() const = 0;
   virtual double duration() const = 0;

third_party/blink/renderer/modules/mediasource/media_source_impl.cc

@@ -401,10 +401,11 @@ void MediaSourceImpl::Trace(Visitor* visitor) {
   ExecutionContextLifecycleObserver::Trace(visitor);
 }
 
-void MediaSourceImpl::SetWebMediaSourceAndOpen(
+void MediaSourceImpl::CompleteAttachingToMediaElement(
     std::unique_ptr<WebMediaSource> web_media_source) {
-  TRACE_EVENT_NESTABLE_ASYNC_END0("media", "MediaSourceImpl::attachToElement",
-                                  TRACE_ID_LOCAL(this));
+  TRACE_EVENT_NESTABLE_ASYNC_END0(
+      "media", "MediaSourceImpl::StartAttachingToMediaElement",
+      TRACE_ID_LOCAL(this));
   DCHECK(web_media_source);
   DCHECK(!web_media_source_);
   DCHECK(attached_element_);
@@ -843,14 +844,15 @@ void MediaSourceImpl::Close() {
   SetReadyState(ClosedKeyword());
 }
 
-bool MediaSourceImpl::AttachToElement(HTMLMediaElement* element) {
+bool MediaSourceImpl::StartAttachingToMediaElement(HTMLMediaElement* element) {
   if (attached_element_)
     return false;
 
   DCHECK(IsClosed());
 
-  TRACE_EVENT_NESTABLE_ASYNC_BEGIN0("media", "MediaSourceImpl::attachToElement",
-                                    TRACE_ID_LOCAL(this));
+  TRACE_EVENT_NESTABLE_ASYNC_BEGIN0(
+      "media", "MediaSourceImpl::StartAttachingToMediaElement",
+      TRACE_ID_LOCAL(this));
   attached_element_ = element;
   return true;
 }

third_party/blink/renderer/modules/mediasource/media_source_impl.h

@@ -94,8 +94,9 @@ class MediaSourceImpl final : public EventTargetWithInlineData,
   static bool isTypeSupported(const String& type);
 
   // html/media/MediaSource interface implementation
-  bool AttachToElement(HTMLMediaElement*) override;
-  void SetWebMediaSourceAndOpen(std::unique_ptr<WebMediaSource>) override;
+  bool StartAttachingToMediaElement(HTMLMediaElement*) override;
+  void CompleteAttachingToMediaElement(
+      std::unique_ptr<WebMediaSource>) override;
   void Close() override;
   bool IsClosed() const override;
   double duration() const override;