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..d3c03c0c6b597c2b09224ec28a1b1c4e657b1cd3 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::updateTimestampOffsetIfNecessary()
+{
+    double offset = m_webSourceBuffer->updatedTimestampOffset();
+    if (!std::isnan(offset))
+        m_timestampOffset = offset;
+}
+
 void SourceBuffer::appendBufferInternal(const unsigned char* data, unsigned size, ExceptionState& exceptionState)
 {
     // Section 3.2 appendBuffer()
@@ -536,6 +543,10 @@ void SourceBuffer::appendBufferAsyncPart()
     }
     m_webSourceBuffer->append(m_pendingAppendData.data(), appendSize);
 
+    // Update our cached |m_timestampOffset| since the implementation may change
+    // it during append() processing.
+    updateTimestampOffsetIfNecessary();
+
     // 3. Set the updating attribute to false.
     m_updating = false;
     m_pendingAppendData.clear();
@@ -690,6 +701,10 @@ void SourceBuffer::didReceiveDataForClient(const char* data, unsigned dataLength
     ASSERT(m_loader);
 
     m_webSourceBuffer->append(reinterpret_cast<const unsigned char*>(data), dataLength);
+
+    // Update our cached |m_timestampOffset| since the implementation may change
+    // it during append() processing.
+    updateTimestampOffsetIfNecessary();
 }
 
 void SourceBuffer::didFinishLoading()