Move HTMLTagCollection to its own file

Move HTMLTagCollection to its own file for clarity, instead of having it
in TagCollection.h. There is no functional change in this CL.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/355163004

git-svn-id: svn://svn.chromium.org/blink/trunk@177232 bbb929c8-8fbe-4397-9dbb-9b2b20218538