Simply FrameLoaderStateMachine and FrameLoader::transitionToCommitted().

Source/core/loader/DocumentLoader.cpp

Index: Source/core/loader/DocumentLoader.cpp
diff --git a/Source/core/loader/DocumentLoader.cpp b/Source/core/loader/DocumentLoader.cpp
index 0b080c310d4a2260d2b7812869025240bfece397..7b4dc65b2e8a337ba050358080be50ace4acee6c 100644
--- a/Source/core/loader/DocumentLoader.cpp
+++ b/Source/core/loader/DocumentLoader.cpp
@@ -298,6 +298,7 @@ void DocumentLoader::commitIfReady()
     if (!m_committed) {
         m_committed = true;
         frameLoader()->commitProvisionalLoad();
+        m_writer.setMIMEType(m_response.mimeType());
     }
 }
 
@@ -681,6 +682,8 @@ void DocumentLoader::commitData(const char* bytes, size_t length)
 
         if (frameLoader()->stateMachine()->creatingInitialEmptyDocument())
             return;
+        if (frameLoader()->stateMachine()->isDisplayingInitialEmptyDocument())
+            frameLoader()->stateMachine()->advanceTo(FrameLoaderStateMachine::CommittedFirstRealLoad);
         
         // The origin is the MHTML file, we need to set the base URL to the document encoded in the MHTML so
         // relative URLs are resolved properly.