Add a missing null check in SliderThumbElement::stopDragging()

hostInput() is null checked in several other SliderThumbElement
functions and clearly can return null, so just handle it.

Add GCController.collect() to the test case to make the crash trigger
reliably, which was necessary in order to debug it.

BUG=356148

Review URL: https://codereview.chromium.org/211403003

git-svn-id: svn://svn.chromium.org/blink/trunk@170139 bbb929c8-8fbe-4397-9dbb-9b2b20218538