[blink] Erase InCustomSpellcheckDictionary marker on spellcheck results

This patch changes spellcheck requester behavior to also erase
InCustomSpellcheckDictionary marker when it receives the spellcheck
results back from the client (the browser). Before this patch, the
requester erased only Spelling and Grammar markers.

This patch is one of several for issue 295190.

TEST=WebFrameTest.SpellcheckResultErasesMarkers
BUG=295190

Review URL: https://chromiumcodereview.appspot.com/24582006

git-svn-id: svn://svn.chromium.org/blink/trunk@158494 bbb929c8-8fbe-4397-9dbb-9b2b20218538