Enable round-tripping and updating of SourceBuffer timestamp offset

Source/modules/mediasource/SourceBuffer.cpp

Index: Source/modules/mediasource/SourceBuffer.cpp
diff --git a/Source/modules/mediasource/SourceBuffer.cpp b/Source/modules/mediasource/SourceBuffer.cpp
index 3f9bbf97b7ac9d4c9188c45eccb459d9880f1c03..a87c7ddb7ea001f929ba153a32f453ca444d2b9a 100644
--- a/Source/modules/mediasource/SourceBuffer.cpp
+++ b/Source/modules/mediasource/SourceBuffer.cpp
@@ -483,6 +483,13 @@ void SourceBuffer::scheduleEvent(const AtomicString& eventName)
     m_asyncEventQueue->enqueueEvent(event.release());
 }
 
+void SourceBuffer::appendAndUpdateTimestampOffset(const unsigned char* data, unsigned length)
+{
+    double newTimestampOffset;
+    if (m_webSourceBuffer->append(data, length, &newTimestampOffset))
+        m_timestampOffset = newTimestampOffset;
+}
+
 void SourceBuffer::appendBufferInternal(const unsigned char* data, unsigned size, ExceptionState& exceptionState)
 {
     // Section 3.2 appendBuffer()
@@ -534,7 +541,7 @@ void SourceBuffer::appendBufferAsyncPart()
         // so that it can clear its end of stream state if necessary.
         m_pendingAppendData.resize(1);
     }
-    m_webSourceBuffer->append(m_pendingAppendData.data(), appendSize);
+    appendAndUpdateTimestampOffset(m_pendingAppendData.data(), appendSize);
 
     // 3. Set the updating attribute to false.
     m_updating = false;
@@ -689,7 +696,7 @@ void SourceBuffer::didReceiveDataForClient(const char* data, unsigned dataLength
     ASSERT(m_updating);
     ASSERT(m_loader);
 
-    m_webSourceBuffer->append(reinterpret_cast<const unsigned char*>(data), dataLength);
+    appendAndUpdateTimestampOffset(reinterpret_cast<const unsigned char*>(data), dataLength);
 }
 
 void SourceBuffer::didFinishLoading()