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..a38d613ae96d9ccc7917c8dbfceff138eba4bfe4 100644
--- a/Source/wtf/text/WTFString.h
+++ b/Source/wtf/text/WTFString.h
@@ -31,6 +31,7 @@
 #include "wtf/text/StringImpl.h"
 #include "wtf/text/StringView.h"
 #include <algorithm>
+#include <iosfwd>
 
 #ifdef __OBJC__
 #include <objc/objc.h>
@@ -633,6 +634,9 @@ 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_EXPORT std::ostream& operator<<(std::ostream&, const String&);

[dcheng, 2015/07/27 19:59:04]

It feels a little sketchy to declare an exported function in one component and
define it in another, even if it does compile.

An alternative I explored was using PrintTo(), but unfortunately, that means
things like:

EXPECT_EQ(a, b) << "more info about b: " << b->fieldThatReturnsString();

need to be rewritten to:
EXPECT_EQ(a, b) << "more info about b: " <<
::testing::PrintToString(b->fieldThatReturnsString());

Ultimately, this felt like the least bad solution.

+
 } // namespace WTF
 
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(String);