[XHR] Delete an incorrect ASSERT

Resource::revalidationSucceeded ASSERTs that the resource exists in the
memory cache. But the assumption is not correct because an ongoing revalidation
request can be evicted by the reloading request for the same URL.
Because no one relies on the assumption, the assert can be deleted.

BUG=370792
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/268743003

git-svn-id: svn://svn.chromium.org/blink/trunk@173711 bbb929c8-8fbe-4397-9dbb-9b2b20218538