Initialize m_isDeletionScheduled

Initialization of m_isDeletionScheduled was missing from one
constructor.

BUG=389216

Review URL: https://codereview.chromium.org/391683003

git-svn-id: svn://svn.chromium.org/blink/trunk@178347 bbb929c8-8fbe-4397-9dbb-9b2b20218538