Oilpan: Do not touch executionContext() in ~DatabaseSync.

ExecutionContext is on-heap and it might be destructed before a DatabaseSync.

BUG=347902

Review URL: https://codereview.chromium.org/376883002

git-svn-id: svn://svn.chromium.org/blink/trunk@177672 bbb929c8-8fbe-4397-9dbb-9b2b20218538