Fix MediaSource.duration setter behavior to match the current spec.

Source/modules/mediasource/MediaSource.cpp

Index: Source/modules/mediasource/MediaSource.cpp
diff --git a/Source/modules/mediasource/MediaSource.cpp b/Source/modules/mediasource/MediaSource.cpp
index 2185806ade015d07eedd1cf3e321640083658c9a..7a427438b4ecd38a6286e44fe92f4dcfafd3c96e 100644
--- a/Source/modules/mediasource/MediaSource.cpp
+++ b/Source/modules/mediasource/MediaSource.cpp
@@ -359,10 +359,37 @@ void MediaSource::setDuration(double duration, ExceptionState& exceptionState)
 
     // 4. Run the duration change algorithm with new duration set to the value being
     // assigned to this attribute.
-    // Synchronously process duration change algorithm to enforce any required
-    // seek is started prior to returning.
-    m_attachedElement->durationChanged(duration);
-    m_webMediaSource->setDuration(duration);
+    durationChangeAlgorithm(duration);
+}
+
+void MediaSource::durationChangeAlgorithm(double newDuration)
+{
+    // Section 2.6.4 Duration change
+    // https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html#duration-change-algorithm
+    // 1. If the current value of duration is equal to new duration, then return.
+    if (newDuration == duration())
+        return;
+
+    // 2. Set old duration to the current value of duration.
+    double oldDuration = duration();
+
+    bool requestSeek = m_attachedElement->currentTime() > newDuration;
+
+    // 3. Update duration to new duration.
+    m_webMediaSource->setDuration(newDuration);
+
+    // 4. If the new duration is less than old duration, then call remove(new duration, old duration) on all all objects in sourceBuffers.
+    if (newDuration < oldDuration) {
+        for (size_t i = 0; i < m_sourceBuffers->length(); ++i)
+            m_sourceBuffers->item(i)->remove(newDuration, oldDuration, ASSERT_NO_EXCEPTION);
+    }
+
+    // 5. If a user agent is unable to partially render audio frames or text cues that start before and end after the duration, then run the following steps:
+    // NOTE: Currently we assume that the media engine is able to render partial frames/cues. If a media
+    // engine gets added that doesn't support this, then we'll need to add logic to handle the substeps.
+
+    // 6. Update the media controller duration to new duration and run the HTMLMediaElement duration change algorithm.
+    m_attachedElement->durationChanged(newDuration, requestSeek);
 }
 
 void MediaSource::setReadyState(const AtomicString& state)