Don't reset LoadEventProgress if frame unload has already started.

third_party/WebKit/Source/core/frame/LocalFrame.cpp

Index: third_party/WebKit/Source/core/frame/LocalFrame.cpp
diff --git a/third_party/WebKit/Source/core/frame/LocalFrame.cpp b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
index 9d4064262a3aae427d5fd76f81870857786f393a..a21ee55cefeae63b41c8150b868fae84149c5f24 100644
--- a/third_party/WebKit/Source/core/frame/LocalFrame.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
@@ -320,6 +320,10 @@ void LocalFrame::detach(FrameDetachType type)
     // detached, so protect a reference to it.
     RefPtrWillBeRawPtr<LocalFrame> protect(this);
     m_loader.stopAllLoaders();
+    // Don't allow any new child frames to load in this frame: attaching a new
+    // child frame during or after detaching children results in an attached
+    // frame on a detached DOM tree, which is bad.
+    SubframeLoadingDisabler disabler(*document());
     m_loader.dispatchUnloadEvent();
     detachChildren();
     m_frameScheduler.clear();