MSE: DCHECK that MediaSourceRegistry |url| params are non-empty

Improves MediaSourceRegistry code readability by DCHECKing in its
URLRegistry overrides and in its LookupMediaSource interface that the
KURL |url| in the former, and the wtf::String |url| in the latter are
not empty (or null). The former is prevented by the FILE API
PublicUrlManager already, and the latter is now (by this change)
protected against in MediaSourceAttachment::LookupMediaSource().

This is a follow-up on a review comment in an MSE-in-Worker CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2300804/7/third_party/blink/renderer/modules/mediasource/media_source_registry_impl.cc#74

BUG=878133

Change-Id: I2ef93b8d3fc894ee71b1a605acdc58df88e722d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346797
Auto-Submit: Matthew Wolenetz <wolenetz@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797640}

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

@@ -24,7 +24,7 @@ MediaSource* MediaSourceAttachment::LookupMediaSource(const String& url) {
   // The only expected caller is an HTMLMediaElement on the main thread.
   DCHECK(IsMainThread());
 
-  if (!registry_)
+  if (!registry_ || url.IsEmpty())
     return nullptr;
 
   // This cast is safe because the only setter of |registry_| is SetRegistry().

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

@@ -18,6 +18,10 @@ namespace blink {
 // scoped_refptr.
 class CORE_EXPORT MediaSourceRegistry : public URLRegistry {
  public:
+  // Finds the attachment, if any, registered with |url| in the
+  // MediaSourceRegistry implementation. |url| must be non-empty. If such an
+  // active registration for |url| is not found, returns an unset
+  // scoped_refptr<MediaSourceAttachment>.
   virtual scoped_refptr<MediaSourceAttachment> LookupMediaSource(
       const String& url) = 0;
 };

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

@@ -47,6 +47,7 @@ void MediaSourceRegistryImpl::RegisterURL(SecurityOrigin*,
                                           URLRegistrable* registrable) {
   DCHECK(IsMainThread());
   DCHECK_EQ(&registrable->Registry(), this);
+  DCHECK(!url.IsEmpty());  // Caller of interface should already enforce this.
 
   DVLOG(1) << __func__ << " url=" << url;
 
@@ -58,6 +59,7 @@ void MediaSourceRegistryImpl::RegisterURL(SecurityOrigin*,
 void MediaSourceRegistryImpl::UnregisterURL(const KURL& url) {
   DVLOG(1) << __func__ << " url=" << url;
   DCHECK(IsMainThread());
+  DCHECK(!url.IsEmpty());  // Caller of interface should already enforce this.
 
   auto iter = media_sources_.find(url.GetString());
   if (iter == media_sources_.end())
@@ -71,8 +73,8 @@ void MediaSourceRegistryImpl::UnregisterURL(const KURL& url) {
 scoped_refptr<MediaSourceAttachment> MediaSourceRegistryImpl::LookupMediaSource(
     const String& url) {
   DCHECK(IsMainThread());
-  return url.IsNull() ? scoped_refptr<MediaSourceAttachment>()
-                      : media_sources_.at(url);
+  DCHECK(!url.IsEmpty());
+  return media_sources_.at(url);
 }
 
 MediaSourceRegistryImpl::MediaSourceRegistryImpl() {

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

@@ -39,7 +39,7 @@ class MediaSourceRegistryImpl final : public MediaSourceRegistry {
 
   // MediaSourceRegistry override that finds |url| in |media_sources_| and
   // returns the corresponding scoped_refptr if found. Otherwise, returns an
-  // unset scoped_refptr.
+  // unset scoped_refptr. |url| must be non-empty.
   scoped_refptr<MediaSourceAttachment> LookupMediaSource(
       const String& url) override;