Mixed Content: Don't override a request's context during redirects.

After https://codereview.chromium.org/605103003/, we have a request
context and frame type for incoming requests generated during redirects.
This patch removes the 'RequestContextInternal' override that was in
place, as it's now actively harmful.

The test was provided by mmaliszkiewicz@opera.com.

BUG=417841

Review URL: https://codereview.chromium.org/608733002

git-svn-id: svn://svn.chromium.org/blink/trunk@182940 bbb929c8-8fbe-4397-9dbb-9b2b20218538