IndexedDB: Ensure IDBRequest.result throws after cursor is continued

IDBRequest.result is cached by the binding layer, with a dirty flag
that is poked when the result changes. A case was missed, and the flag
was not poked when request is re-used by a continued cursor, so
the cached value was returned instead of an exception being raised.

Clear the result via the setResult() helper which pokes the dirty
flag.

BUG=331132
R=dgrogan

Review URL: https://codereview.chromium.org/120853003

git-svn-id: svn://svn.chromium.org/blink/trunk@164427 bbb929c8-8fbe-4397-9dbb-9b2b20218538