Remove unneeded RELEASE_ASSERT in FrameLoader

This was used to try to identify the source of re-entrancy that was
causing FrameLoaderClient to be unexpectedly null. The source has since
been identified, so this code is no longer needed.

BUG=363099,371084

Review URL: https://codereview.chromium.org/506783004

git-svn-id: svn://svn.chromium.org/blink/trunk@180919 bbb929c8-8fbe-4397-9dbb-9b2b20218538