Retry: Add some DVLOG's in MetadataDatabase::Update* methods

Added a forward-decl since the original patch.
(CQ doesn't catch the breakage, hopefully this one can make it)

BUG=none
TEST=manual
TBR=tzik,nhiroki

Review URL: https://codereview.chromium.org/101543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238613 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238613