Cleanup: Remove WebPlatformDatabaseObserver typedef and temporary header.

kinuko has switched over to WebDatabaseObserver in
https://src.chromium.org/viewvc/chrome?view=rev&revision=235714

BUG=None
TEST=None
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/97923002

git-svn-id: svn://svn.chromium.org/blink/trunk@163123 bbb929c8-8fbe-4397-9dbb-9b2b20218538