fix testing/runner/SpellCheckClient leak when multiple spellcheck requests are queued.

Before this patch, the SpellCheckClient did not cancel nor complete the previous spellcheck request before processing new one, causing a leak.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/23133005

git-svn-id: svn://svn.chromium.org/blink/trunk@156190 bbb929c8-8fbe-4397-9dbb-9b2b20218538