Followup cleanup for WebDatabaseObserver

- Rename WebPlatformDatabaseObserver back to WebDatabaseObserver

Followup patch for
https://src.chromium.org/viewvc/chrome?revision=234602&view=revision
(Change WebDatabaseObserverImpl to implement public/platform's
WebDatabaseObserver)

BUG=none
R=jam@chromium.org, jam

Review URL: https://codereview.chromium.org/74043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235714 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235714