Protect contextElement during insertAdjacentHTML call

JS event handlers may cause element to lose its last ref during
parsing.

BUG=315842

Review URL: https://codereview.chromium.org/67813002

git-svn-id: svn://svn.chromium.org/blink/trunk@161697 bbb929c8-8fbe-4397-9dbb-9b2b20218538