MediaQueryTokenizer does not parse correctly negative exponents

MediaQueryTokenizer does not parse correctly negative exponents. The sign
parameter for getExponent() was passed by value instead of reference.
Therefore, setting the sign to -1 in the function had no impact on the
caller.

TEST=MediaQueryTokenizerTest

Review URL: https://codereview.chromium.org/238263008

git-svn-id: svn://svn.chromium.org/blink/trunk@171836 bbb929c8-8fbe-4397-9dbb-9b2b20218538