Tidy DEFINE_(THREAD_SAFE_)STATIC_LOCAL() implementations.

third_party/WebKit/Source/platform/heap/Member.h

Index: third_party/WebKit/Source/platform/heap/Member.h
diff --git a/third_party/WebKit/Source/platform/heap/Member.h b/third_party/WebKit/Source/platform/heap/Member.h
index b2bac898b0c9e4b65564bf2e4e3e53dca4d4bd83..5ed599b791d91c9b2af3f63b09d7c6df1942c9f7 100644
--- a/third_party/WebKit/Source/platform/heap/Member.h
+++ b/third_party/WebKit/Source/platform/heap/Member.h
@@ -13,6 +13,7 @@ namespace blink {
 
 template <typename T>
 class Persistent;
+class ScriptWrappable;
 template <typename T>
 class TraceWrapperMember;
 
@@ -172,6 +173,14 @@ class MemberBase {
       // collection iterator.
       DCHECK(m_creationThreadState || !m_raw);
     }
+
+    // If this DCHECK() triggers, you're attempting to embed a ScriptWrappable
+    // inside a static local singleton, which is unsafe.
+    // See |DEFINE_STATIC_LOCAL()| documentation for further details.
+    if (WTF::IsSubclass<T, blink::ScriptWrappable>::value) {
+      if (m_creationThreadState)
+        DCHECK(m_creationThreadState->canAllocateScriptWrappable());

[haraken, 2017/02/11 10:25:20]

Would you help me understand why we need to have the check here? i.e., why is
the check in ScriptWrappable's constructor not enough?

[sof, 2017/02/11 12:09:11]

Good question to raise -- the "no ScriptWrappable" scope is something we set up
while the singleton is constructed. So, in order to detect the fields referring
to ScriptWrappables that aren't eagerly initialized, we fall back to the
MemberBase<> check above for the *Member<> fields that are default initialized.

Which is of course only an approximation, as ScriptWrappables may reside deeper
within the object type. I should document that more clearly, if we end up taking
this on..

The other original suggestion of using the type information that GC plugin has
to do this detection, would be accurate. But that might generate false negatives
while computing ScriptWrappable reachability (e.g., should we trace through a
LocalFrame* reference or not.)

+    }
 #endif
   }