[base] Inline AssertIteratorsInOrder in StringPiece

This change inlines the definition of AssertIteratorsInOrder into the
string_piece.h. While the goal of the previous implementation was to
avoid #including "base/logging.h", that header has already been included
with r353605, thus there should be no regressions when inlining this
definition. Having AssertIteratorsInOrder inline also directly supports
BasicStringPieces that are not base::StringPiece or base::StringPiece16.

Bug: 911896
Change-Id: I9150235dc79687b72f970872af950b0a82e1c0a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019442
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735055}

base/strings/string_piece.cc

@@ -438,16 +438,5 @@ StringPiece16 substr(const StringPiece16& self,
   return substrT(self, pos, n);
 }
 
-#if DCHECK_IS_ON()
-void AssertIteratorsInOrder(std::string::const_iterator begin,
-                            std::string::const_iterator end) {
-  DCHECK(begin <= end) << "StringPiece iterators swapped or invalid.";
-}
-void AssertIteratorsInOrder(string16::const_iterator begin,
-                            string16::const_iterator end) {
-  DCHECK(begin <= end) << "StringPiece iterators swapped or invalid.";
-}
-#endif
-
 }  // namespace internal
 }  // namespace base

base/strings/string_piece.h

@@ -141,14 +141,6 @@ BASE_EXPORT StringPiece16 substr(const StringPiece16& self,
                                  size_t pos,
                                  size_t n);
 
-#if DCHECK_IS_ON()
-// Asserts that begin <= end to catch some errors with iterator usage.
-BASE_EXPORT void AssertIteratorsInOrder(std::string::const_iterator begin,
-                                        std::string::const_iterator end);
-BASE_EXPORT void AssertIteratorsInOrder(string16::const_iterator begin,
-                                        string16::const_iterator end);
-#endif
-
 }  // namespace internal
 
 // BasicStringPiece ------------------------------------------------------------
@@ -189,11 +181,7 @@ template <typename STRING_TYPE> class BasicStringPiece {
       : ptr_(offset), length_(len) {}
   BasicStringPiece(const typename STRING_TYPE::const_iterator& begin,
                    const typename STRING_TYPE::const_iterator& end) {
-#if DCHECK_IS_ON()
-    // This assertion is done out-of-line to avoid bringing in logging.h and
-    // instantiating logging macros for every instantiation.
-    internal::AssertIteratorsInOrder(begin, end);
-#endif
+    DCHECK(begin <= end) << "StringPiece iterators swapped or invalid.";
     length_ = static_cast<size_t>(std::distance(begin, end));
 
     // The length test before assignment is to avoid dereferencing an iterator