Invalid chars should not be allowed as first character in custom element name

This patch *disallows* usage of remaining combining chars as first
character in custom element name. Also, usage of colon (":") is not
allowed in custom element name. This restriction is according to the
NCName production mentioned in the specs.

BUG=325673
TEST=fast/dom/custom/invalid-first-char-combinators.html

Review URL: https://codereview.chromium.org/493713002

git-svn-id: svn://svn.chromium.org/blink/trunk@184957 bbb929c8-8fbe-4397-9dbb-9b2b20218538