Fix shadowing warning in json_parser on VS2015

d:\src\cr3\src\base\json\json_parser.cc(431): error C2220: warning treated as error - no 'object' file generated
d:\src\cr3\src\base\json\json_parser.cc(431): warning C4456: declaration of 'next_char' hides previous local declaration
d:\src\cr3\src\base\json\json_parser.cc(427): note: see declaration of 'next_char'

I don't think there's any need for a new variable inside the loop.

R=thestig@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/881003003

Cr-Commit-Position: refs/heads/master@{#313374}

base/json/json_parser.cc

@@ -428,7 +428,7 @@ bool JSONParser::EatComment() {
   if (next_char == '/') {
     // Single line comment, read to newline.
     while (CanConsume(1)) {
-      char next_char = *NextChar();
+      next_char = *NextChar();
       if (next_char == '\n' || next_char == '\r')
         return true;
     }