Refactoring: Delete DatabaseBackendContext

This change merges DatabaseBackendContext to DatabaseContext as
the former is doing almost nothing.

This also eliminates dependency from core/ to DatabaseManager.h.
The dependency was there just because keeping the context's lifetime
which can be implemented using a LifecycleObserver callback.

BUG=none
TEST=none
R=abarth

Review URL: https://codereview.chromium.org/27483004

git-svn-id: svn://svn.chromium.org/blink/trunk@159778 bbb929c8-8fbe-4397-9dbb-9b2b20218538