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 c5dbd81bee9313d410e27146666535a8dd3e82f3..7dd228d5128c286fd97025dac17c788807b6b515 100644
--- a/Source/core/loader/DocumentLoader.cpp
+++ b/Source/core/loader/DocumentLoader.cpp
@@ -292,6 +292,7 @@ void DocumentLoader::commitIfReady()
     if (!m_committed) {
         m_committed = true;
         frameLoader()->commitProvisionalLoad();
+        m_writer.setMIMEType(m_response.mimeType());
     }
 }
 
@@ -646,6 +647,8 @@ 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.