Fix template bugs that prevent rolling gtest DEPS in Chrome.

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index ed9bb1256b950271dafaead45dfcaa11781408cb..7a701fc38f7ad9416b1c05669ead56532ea3fc4c 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -139,12 +139,16 @@ public:
     static const bool value = false;
 };
 
+template <typename T> const bool NeedsAdjustAndMark<T, true>::value;

[Ken Russell (switch to Gerrit), 2014/07/30 22:21:29]

Why is this out-of-line definition needed? Will this cause the variable to be
instantiated into every translation unit which includes this header?

[Nico, 2014/07/30 22:26:02]

Yes, that seems wrong.

[Nick Bray (chromium), 2014/07/30 22:33:45]

It isn't technically "out-of-line" because the inline definition is incomplete
on its own.  static variables in classes are essentially an "extern" declaration
for storage that is declared elsewhere.  Normally that "elsewhere" is in a .cc
file, but for templates you don't want to declare the storage for each concrete
instantiation.  Declaring it in a header file would normally cause a multiple
definition problem because each source file that included the header file would
create its own version, but C++ cheats and has a "one definition rule" that
covers this case.  The templated instantiations of this variable are stored in
the COMDAT section and deduplicated, similar to inline method definitions in a
header file.

http://stackoverflow.com/questions/1834597/what-is-the-comdat-section-used-for

Without this declaration, compilers _may_ be able to const fold accesses to this
field out of existence before the link step, so the missing storage declaration
is not noticed.  No such luck with gtest, however, there was a link error.

+
 template<typename T>
 class NeedsAdjustAndMark<T, false> {
 public:
     static const bool value = WTF::IsSubclass<typename WTF::RemoveConst<T>::Type, GarbageCollectedMixin>::value;
 };
 
+template <typename T> const bool NeedsAdjustAndMark<T, false>::value;
+
 template<typename T, bool = NeedsAdjustAndMark<T>::value> class DefaultTraceTrait;
 
 // The TraceTrait is used to specify how to mark an object pointer and