Restore PendingScript prefinalizer.

Under some as yet unknown conditions, PendingScript objects can become garbage
without having been explicitly detached/disposed of first. Hence restore the
prefinalizer that r396656 removed.

R=
BUG=615977

Review-Url: https://codereview.chromium.org/2021773004
Cr-Commit-Position: refs/heads/master@{#396813}

third_party/WebKit/Source/core/dom/PendingScript.cpp

@@ -47,12 +47,11 @@ PendingScript::PendingScript(Element* element, ScriptResource* resource)
     , m_client(nullptr)
 {
     setScriptResource(resource);
+    ThreadState::current()->registerPreFinalizer(this);
 }
 
 PendingScript::~PendingScript()
 {
-    // Verify explicit dispose().
-    CHECK(!m_client && !m_element && !m_streamer);
 }
 
 void PendingScript::dispose()

third_party/WebKit/Source/core/dom/PendingScript.h

@@ -47,6 +47,7 @@ class ScriptSourceCode;
 // lifetime in order to guarantee that the data buffer will not be purged.
 class CORE_EXPORT PendingScript final : public GarbageCollectedFinalized<PendingScript>, public ResourceOwner<ScriptResource> {
     USING_GARBAGE_COLLECTED_MIXIN(PendingScript);
+    USING_PRE_FINALIZER(PendingScript, dispose);
     WTF_MAKE_NONCOPYABLE(PendingScript);
 public:
     static PendingScript* create(Element*, ScriptResource*);