Improve GIF image decoding by using Vector<>::iterator

Vector<>::operator[] does an implicit bounds check in Blink's implementation of
vector. Using iterator to access intermediate buffers used for LZW
decoding reduces processing time by 12%.

Also uses fixed-size arrays for |prefix|, |suffix| and |stack|.

BUG=233076

Review URL: https://chromiumcodereview.appspot.com/20137002

git-svn-id: svn://svn.chromium.org/blink/trunk@154970 bbb929c8-8fbe-4397-9dbb-9b2b20218538