Fix Blink commit type for subframes after initial about:blank load.

Source/core/loader/FrameLoader.cpp

Index: Source/core/loader/FrameLoader.cpp
diff --git a/Source/core/loader/FrameLoader.cpp b/Source/core/loader/FrameLoader.cpp
index e63eca19569d0a418ff100f0d677b7740187d16d..6c75488235f85100a16a8b2314d583b8d89ac980 100644
--- a/Source/core/loader/FrameLoader.cpp
+++ b/Source/core/loader/FrameLoader.cpp
@@ -675,6 +675,15 @@ FrameLoadType FrameLoader::determineFrameLoadType(const FrameLoadRequest& reques
 
 bool FrameLoader::prepareRequestForThisFrame(FrameLoadRequest& request)
 {
+    // From the HTML5 spec for location.assign():

[Charlie Reis, 2015/06/09 07:14:18]

I'm moving this rule from NavigationScheduler::mustLockBackForwardList and not
the rest, since the other parts don't work at this point in the timeline.  (They
depend on whether a load is in progress, which is always true here for this
request, regardless of what the state was at the start of the request.)

+    //  "If the browsing context's session history contains only one Document,
+    //   and that was the about:blank Document created when the browsing context
+    //   was created, then the navigation must be done with replacement enabled."
+    if (!m_stateMachine.committedMultipleRealLoads()

[Nate Chapin, 2015/06/09 18:25:33]

Should this be a case in determineFrameLoadType()?

[Charlie Reis, 2015/06/11 16:43:48]

Yes, that seems reasonable.  Done.

+        && equalIgnoringCase(m_frame->document()->url(), blankURL())) {
+        request.setLockBackForwardList(true);
+    }
+
     // If no origin Document* was specified, skip remaining security checks and assume the caller has fully initialized the FrameLoadRequest.
     if (!request.originDocument())
         return true;