[blink] Rename InCustomSpellcheckDictioanry to InvisibleSpellcheck

This patch renames InCustomSpellcheckDictioanry markers and decorations to
InvisibleSpellcheck. This change hides the intent of the browser in adding
invisible text markers to the text. The resulting markers are more
explicit in their look: they are invisible. The marker purpose should also
be clear: they are for the spellcheck client.

This patch prevents adding multiple invisible marker types. For example,
after this patch we no longer need to add WasMisspelled marker.

This patch is the result of discussion about marker and decoration names
on https://codereview.chromium.org/25740003.

BUG=302807

Review URL: https://codereview.chromium.org/25413004

git-svn-id: svn://svn.chromium.org/blink/trunk@159045 bbb929c8-8fbe-4397-9dbb-9b2b20218538