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..55f39465a8c59abdcd96ac7fa5425a804aad10e8 100644
--- a/third_party/WebKit/Source/core/frame/LocalFrame.cpp
+++ b/third_party/WebKit/Source/core/frame/LocalFrame.cpp
@@ -320,6 +320,14 @@ 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.
+    // TODO(dcheng): Strictly speaking, this only needs to be scoped immediately
+    // before detachChildren(). However, this was previously scoped to the
+    // unload event as well. Investigate if it's possible to move it after
+    // dispatchUnloadEvent(), and if it makes sense to do that.
+    SubframeLoadingDisabler disabler(*document());
     m_loader.dispatchUnloadEvent();
     detachChildren();
     m_frameScheduler.clear();