Use early-return style in SpellChecker::spellCheckOldSelection()

This patch makes |SpellChecker::spellCheckOldSelection()| to use early-return
style to reduce indentation for improving readability.

BUG=n/a
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2123863003
Cr-Commit-Position: refs/heads/master@{#404080}
1 file changed