Fix ODR violation that can break WTF::String pretty printing in tests.

Source/wtf/text/WTFString.h

Index: Source/wtf/text/WTFString.h
diff --git a/Source/wtf/text/WTFString.h b/Source/wtf/text/WTFString.h
index d6c82501417f8605de3c1f086aac5c888192be9b..f06ed79dbc305cce03387bb3d2eddf138aa33bc5 100644
--- a/Source/wtf/text/WTFString.h
+++ b/Source/wtf/text/WTFString.h
@@ -27,10 +27,12 @@
 
 #include "wtf/HashTableDeletedValueType.h"
 #include "wtf/WTFExport.h"
+#include "wtf/testing/WTFUnitTestHelpersExport.h"
 #include "wtf/text/ASCIIFastPath.h"
 #include "wtf/text/StringImpl.h"
 #include "wtf/text/StringView.h"
 #include <algorithm>
+#include <iosfwd>
 
 #ifdef __OBJC__
 #include <objc/objc.h>
@@ -633,6 +635,9 @@ WTF_EXPORT const String& emptyString();
 WTF_EXPORT const String& emptyString16Bit();
 WTF_EXPORT extern const String& xmlnsWithColon;
 
+// Pretty printer for gtest. Declared here to avoid ODR violations.
+WTF_UNITTEST_HELPERS_EXPORT std::ostream& operator<<(std::ostream&, const String&);

[Julien - ping for review, 2015/07/28 17:35:32]

It seems to me that we would benefit from a printer for WTFString so this should
be a member function instead of a free function.

That's something that wasn't done in WebKit land but would be good IMO.

+
 } // namespace WTF
 
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(String);