Move EmptyString, kWhitespace and the BOM to base.

base/strings/string_util.h

Index: base/strings/string_util.h
diff --git a/base/strings/string_util.h b/base/strings/string_util.h
index 2b6316bc5f20fa85c38363a4ac76d5b1a09e1a0c..a6ee81542a96246ac550afb19156207ecc82690c 100644
--- a/base/strings/string_util.h
+++ b/base/strings/string_util.h
@@ -123,34 +123,41 @@ template<typename Char> struct CaseInsensitiveCompareASCII {
   }
 };
 
-}  // namespace base
-
-#if defined(OS_WIN)
-#include "base/strings/string_util_win.h"
-#elif defined(OS_POSIX)
-#include "base/strings/string_util_posix.h"
-#else
-#error Define string operations appropriately for your platform
-#endif
-
 // These threadsafe functions return references to globally unique empty
 // strings.
 //
-// DO NOT USE THESE AS A GENERAL-PURPOSE SUBSTITUTE FOR DEFAULT CONSTRUCTORS.
-// There is only one case where you should use these: functions which need to
-// return a string by reference (e.g. as a class member accessor), and don't
-// have an empty string to use (e.g. in an error case).  These should not be
-// used as initializers, function arguments, or return values for functions
-// which return by value or outparam.
+// It is likely faster to construct a new empty string object (just a few
+// instructions to set the length to 0) than to get the empty string singleton
+// returned by these functions (which requires threadsafe singleton access).
+//
+// Therefore, DO NOT USE THESE AS A GENERAL-PURPOSE SUBSTITUTE FOR DEFAULT
+// CONSTRUCTORS. There is only one case where you should use these: functions
+// which need to return a string by reference (e.g. as a class member
+// accessor), and don't have an empty string to use (e.g. in an error case).
+// These should not be used as initializers, function arguments, or return
+// values for functions which return by value or outparam.
 BASE_EXPORT const std::string& EmptyString();
 BASE_EXPORT const string16& EmptyString16();
 
+// Contains the set of characters representing whitespace in the corresponding
+// encoding. Null-terminated.
 BASE_EXPORT extern const wchar_t kWhitespaceWide[];
 BASE_EXPORT extern const char16 kWhitespaceUTF16[];
 BASE_EXPORT extern const char kWhitespaceASCII[];
 
+// Null-terminated string representing the UTF-8 byte order mark.
 BASE_EXPORT extern const char kUtf8ByteOrderMark[];
 
+}  // namespace base
+
+#if defined(OS_WIN)
+#include "base/strings/string_util_win.h"
+#elif defined(OS_POSIX)
+#include "base/strings/string_util_posix.h"
+#else
+#error Define string operations appropriately for your platform
+#endif
+
 // Removes characters in |remove_chars| from anywhere in |input|.  Returns true
 // if any characters were removed.  |remove_chars| must be null-terminated.
 // NOTE: Safe to use the same variable for both |input| and |output|.
@@ -370,7 +377,7 @@ inline Char HexDigitToInt(Char c) {
 
 // Returns true if it's a whitespace character.
 inline bool IsWhitespace(wchar_t c) {
-  return wcschr(kWhitespaceWide, c) != NULL;
+  return wcschr(base::kWhitespaceWide, c) != NULL;
 }
 
 // Return a byte string in human-readable format with a unit suffix. Not