Remove unused closehelperPlugin()

closehelperPlugin() is removed as closeHelperPluginSoon() should be
used instead. The only caller of closehelperPlugin() is in the same
class and has been changed.

BUG=

Review URL: https://codereview.chromium.org/25666012

git-svn-id: svn://svn.chromium.org/blink/trunk@158880 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix underflow when animating properties of type 'unsigned short'

This affects outline-width, -webkit-column-count and -webkit-column-rule-width.

Note that these were intended to be fixed by
https://chromiumcodereview.appspot.com/23287005, but that change only fixed
properties of type 'unsigned'.

Also templatize the simple blendFunc() functions.

Note that animation of -webkit-column-count is currently broken. See
crbug.com/303444.

BUG=276109
R=dstockwell

Review URL: https://codereview.chromium.org/25753006

git-svn-id: svn://svn.chromium.org/blink/trunk@158881 bbb929c8-8fbe-4397-9dbb-9b2b20218538