Add more context to the pseudoElement tag name.

Currently the tagName for a pseudoElement is <pseudo>. This doesn't help if
there are several pseudo elements in the tree. This CL updates the tagName
to be <pseudo:before> or <pseudo:after> depending on the type of pseudoElement.

Review URL: https://codereview.chromium.org/595763003

git-svn-id: svn://svn.chromium.org/blink/trunk@182601 bbb929c8-8fbe-4397-9dbb-9b2b20218538