base/json: Make ReadHexDigits easier to read by using IsHexDigit() function.

base/json/json_reader.cc

Index: base/json/json_reader.cc
diff --git a/base/json/json_reader.cc b/base/json/json_reader.cc
index d3d712a5b74540e13c63b16b7b568a0f1510914a..748cc4e263f6cd153a0dd725ddb343f391443e10 100644
--- a/base/json/json_reader.cc
+++ b/base/json/json_reader.cc
@@ -53,10 +53,8 @@ bool ReadHexDigits(base::JSONReader::Token& token, int digits) {
     wchar_t c = *(token.begin + token.length + i);
     if ('\0' == c)
       return false;
-    if (!(('0' <= c && c <= '9') || ('a' <= c && c <= 'f') ||
-          ('A' <= c && c <= 'F'))) {
+    if (!IsHexDigit(c))
       return false;
-    }
   }
 
   token.length += digits;