Remove helpless isHTMLFooElement functions.

We can replace them with hasTagName, and hasTagName doesn't need
header includes.

isHTMLFooElement should mean that the specified object is an instance
of C++ HTMLFooElement class. Some modules are not interested in
C++ implementation details, and need to know only tag names.

BUG=

Review URL: https://codereview.chromium.org/93903009

git-svn-id: svn://svn.chromium.org/blink/trunk@164306 bbb929c8-8fbe-4397-9dbb-9b2b20218538