Refactor SpellChecker::respondToChangedSelection

Make variables const and remove |isContinuousGrammarCheckingEnabled|
because it equals to |isContinuousSpellCheckingEnabled|.

TEST=No change in behavior.

Review-Url: https://codereview.chromium.org/2101263002
Cr-Commit-Position: refs/heads/master@{#403646}
1 file changed