[blink] Add a DocumentMarker::MisspellingMarkers helper

This patch adds DocumentMarker::MisspellingMarkers helper to be used instead of
(DocumentMarker::Spelling | DocumentMarker::Grammar) through-out Blink. This
patch is first of two for issue 295190, which aims to add an invisible
DocumentMarker::InCustomSpellcheckDictionary marker to Blink.

BUG=295190

Review URL: https://chromiumcodereview.appspot.com/24198006

git-svn-id: svn://svn.chromium.org/blink/trunk@158125 bbb929c8-8fbe-4397-9dbb-9b2b20218538