Remove WebElement::hasTagName.

There are no callsites in Chromium.  This function is unsafe because it
ignores namespace URI.

BUG=

Review URL: https://codereview.chromium.org/354763002

git-svn-id: svn://svn.chromium.org/blink/trunk@176895 bbb929c8-8fbe-4397-9dbb-9b2b20218538