[webdatabase] protect access to m_terminationRequested with a mutex

It's accessed from the main and the database thread.

BUG=328393
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/110883012

git-svn-id: svn://svn.chromium.org/blink/trunk@164083 bbb929c8-8fbe-4397-9dbb-9b2b20218538