LibWeb: Begin implementing HTMLMediaElement's readyState attribute

It's not totally clear to me when all of these states are supposed to be
set. For example, nothing in the HTMLMediaElement spec says to "set the
readyState attribute to HAVE_ENOUGH_DATA". However, this will at least
advance the readyState to HAVE_METADATA, which is needed for other
useful attributes for debugging.

Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h

@@ -33,6 +33,15 @@ public:
 
     WebIDL::ExceptionOr<Bindings::CanPlayTypeResult> can_play_type(DeprecatedString const& type) const;
 
+    enum class ReadyState : u16 {
+        HaveNothing,
+        HaveMetadata,
+        HaveCurrentData,
+        HaveFutureData,
+        HaveEnoughData,
+    };
+    ReadyState ready_state() const { return m_ready_state; }
+
     void load() const;
     double duration() const;
     void pause() const;
@@ -60,6 +69,7 @@ private:
     WebIDL::ExceptionOr<void> process_media_data(Function<void()> failure_callback);
     WebIDL::ExceptionOr<void> handle_media_source_failure();
     void forget_media_resource_specific_tracks();
+    void set_ready_state(ReadyState);
     void set_duration(double);
 
     // https://html.spec.whatwg.org/multipage/media.html#media-element-event-task-source
@@ -68,6 +78,10 @@ private:
     // https://html.spec.whatwg.org/multipage/media.html#dom-media-networkstate
     NetworkState m_network_state { NetworkState::Empty };
 
+    // https://html.spec.whatwg.org/multipage/media.html#dom-media-readystate
+    ReadyState m_ready_state { ReadyState::HaveNothing };
+    bool m_first_data_load_event_since_load_start { false };
+
     // https://html.spec.whatwg.org/multipage/media.html#dom-media-duration
     double m_duration { NAN };