GCC: fix comparison of base::StringPiece in char_traits

Comparing base::StringPiece("\x11\x11") with
base::StringPiece("\x99\x99") wrongly returns that "\x99\x99" is smaller
than "\x11\x11". The problem here is that characters are interpreted
signed. In previous example 0x99 is therefore interpreted as -104 which
of course is smaller than 17 (0x11). Clang is not affected, because it
uses __builtin_memcmp or __builtin_wmemcmp.

Solution is to use std::char_traits<T>::lt, which is constexpr in C++14
already and handles signed characters correctly.

Bug: 941696
Change-Id: I24edfa4f92058faf87bd029b06832ae1e9e3a6d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624670
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843037}

base/strings/char_traits.h

@@ -7,6 +7,8 @@
 
 #include <stddef.h>
 
+#include <string>
+
 #include "base/compiler_specific.h"
 
 namespace base {
@@ -33,10 +35,14 @@ template <typename T>
 constexpr int CharTraits<T>::compare(const T* s1,
                                      const T* s2,
                                      size_t n) noexcept {
+  // Comparison with operator < fails, because of signed/unsigned
+  // mismatch, https://crbug.com/941696
+  // std::char_traits<T>::lt is guaranteed to be constexpr in C++14:
+  // https://timsong-cpp.github.io/cppwp/n4140/char.traits.specializations#char
   for (; n; --n, ++s1, ++s2) {
-    if (*s1 < *s2)
+    if (std::char_traits<T>::lt(*s1, *s2))
       return -1;
-    if (*s1 > *s2)
+    if (std::char_traits<T>::lt(*s2, *s1))
       return 1;
   }
   return 0;