Remove defunct LSAN leak annotation in ensureDomInJSContext

TBR=earthdok
BUG=328552

Review URL: https://codereview.chromium.org/219673002

git-svn-id: svn://svn.chromium.org/blink/trunk@170465 bbb929c8-8fbe-4397-9dbb-9b2b20218538