spellcheck-editable-on-focus.html does not test all elements

This tests misspellings on editable elements (div, text area, input).
However, after r161840 only first one is being tested.

Additionally, rename setFocusOnEditableElements() to triggerSpellingForEditables()
to better describe what it does.

BUG=

Review URL: https://codereview.chromium.org/74003003

git-svn-id: svn://svn.chromium.org/blink/trunk@162124 bbb929c8-8fbe-4397-9dbb-9b2b20218538