[blink] Add TextDecorationType enum

This patch splits off a part of TextCheckingType enum into
TextDecorationType.

After this patch, TextCheckingType enum denotes the types of spellcheck
that Blink can request from the browser.

The browser uses the TextDecorationType enum to indicate the types of text
decorations that Blink needs to store in text. Some of these decorations
are visible (spelling and grammar). Other decorations are invisible (in
custom spellcheck dictionary).

BUG=302807

Review URL: https://codereview.chromium.org/25740003

git-svn-id: svn://svn.chromium.org/blink/trunk@158971 bbb929c8-8fbe-4397-9dbb-9b2b20218538