Don't force callers to be ASCII (when some of them already aren't).

TBR=tfarina
BUG=none
TEST=none
Review URL: http://codereview.chromium.org/3137015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@56451 0039d316-1c4b-4281-b951-d872f2087c98
   bool SetVarImpl(const char* variable_name, const std::string& new_value) {
 #if defined(OS_POSIX)
    return !setenv(variable_name, new_value.c_str(), 1);
 #elif defined(OS_WIN)
    // On success, a nonzero value is returned.
    return !!SetEnvironmentVariable(UTF8ToWide(variable_name).c_str(),
                                    UTF8ToWide(new_value).c_str());
 #endif
   }

   bool SetVarImpl(const char* variable_name, const std::string& new_value) {
 #if defined(OS_POSIX)
    return setenv(variable_name, new_value.c_str(), 1) == 0;
 #elif defined(OS_WIN)
    return ::SetEnvironmentVariable(ASCIIToWide(variable_name).c_str(),
                                    ASCIIToWide(new_value).c_str()) != 0;
 #endif
   }
