Add explicit destructor to DatabaseBackend.h.

This class was missed in https://codereview.chromium.org/412483007/, see that
CL's description for the motivation.

I filed http://llvm.org/20452 about the diagnostic for this not being as clear
as it could be.

BUG=82385
R=rnk@chromium.org
TBR=michaeln

Review URL: https://codereview.chromium.org/419853002

git-svn-id: svn://svn.chromium.org/blink/trunk@178945 bbb929c8-8fbe-4397-9dbb-9b2b20218538