IndexedDB: Force a database compaction after a database is deleted

The per-origin backing store used by Chromium's Indexed DB - leveldb -
lazily reclaims space. A delete operation actually increases usage by
writing additional "this key is not present" until a compaction
occurs, triggered by various heuristics and likely occurring on a
background thread. This can make even well-intentioned scripts run
into quota errors.

This CL introduces one big hammer for the worst case: when an IDB
database within in an origin is deleted, force a synchronous
compaction.

R=dgrogan
BUG=327993

Review URL: https://codereview.chromium.org/100013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241339 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241339