HTML Imports: Send credentials for same origin requests

This captures following spec chagne: https://www.w3.org/Bugs/Public/show_bug.cgi?id=24905
The essential part of the change is in HTMLImportsController.cpp.
Anything else is to make it work with redirect.

The problem here is that allowCredentials flag is held both
by ResourceLoaderOptions and ResourceRequest and these two
can go out-of-sync. This change tries to make them in sync.

Such a state duplication should be resolved eventually, but
that is another story.

BUG=348671
TEST=import-cors-credentials.html
R=abarth, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/196043002

git-svn-id: svn://svn.chromium.org/blink/trunk@169496 bbb929c8-8fbe-4397-9dbb-9b2b20218538