Add an error page for HTTP 404 error pages without bodies.
We used to show Link Doctor pages in this case, but that broke long ago.
https://crbug.com/331745 was recently fixed, which allows us to show error
pages in that case once more.
Also add a browser test for HTTP 500 errors pages without bodies, which
also fixed by that CL.
Also fixes a bunch of browser tests that implicitly depend on *not* showing
error pages when we get a 404 response without a body.
BUG=632781,331745
Review-Url: https://codereview.chromium.org/2190463008
Cr-Commit-Position: refs/heads/master@{#409567}
18 files changed
