IndexedDB: Remove unused IDBRequest::m_taskType member

This member was unused - the task type is only needed when initializing
the cursor. It could be used instead of m_preventPropagation but it's
not worth the plumbing.

R=alecflett@chromium.org

Review URL: https://codereview.chromium.org/62423004

git-svn-id: svn://svn.chromium.org/blink/trunk@161557 bbb929c8-8fbe-4397-9dbb-9b2b20218538