Spell check whole content of an editable element when it gets focused.

This is how Gecko and Presto behave. Without this patch Blink spell checks
an editable after moving the caret/unfocusing the editable but the check spans
only the line the caret was previously at (possibly plus 1-2 lines around).
Textareas and contenteditables are only spell checked on
first focus.

BUG=264730

Review URL: https://chromiumcodereview.appspot.com/21694005

git-svn-id: svn://svn.chromium.org/blink/trunk@156583 bbb929c8-8fbe-4397-9dbb-9b2b20218538