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WebUINavigationThrottle blocks WebUI (chrome://...) pages from loading
non-WebUI pages in subframes.

When a renderer navigates to a WebUI page, a "WebUI bindings" bit is set
in that renderer's ChildProcessSecurityPolicyImpl::SecurityState, and it
remains for the lifetime of the renderer process.
WebUINavigationThrottles are created for renderers with this bit set.

In multi-process mode, navigating away from the WebUI page creates a
fresh renderer with a fresh SecurityState, so subsequent non-WebUI pages
are again able to load non-WebUI subframes.

In single-process mode, the WebUI bit remains. After visiting a WebUI
page, iframes on all other pages will be broken until Chrome is
restarted.

This breaks Android WebView as well, since WebView uses single-process
on old Android versions and low-memory devices. It breaks CTS because
testOnSafeBrowsingHit loads chrome://safe-browsing/match?type=malware,
which sets the bit, and then testShouldOverrideUrlLoadingOnCreateWindow
tries to use an iframe.

Fix this by not creating a WebUINavigationThrottle in single-process
mode.

BUG=924799

Change-Id: I5a61f45dc065681f0e6b97e0b205e804edbd7402
Reviewed-on: https://chromium-review.googlesource.com/c/1432953
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626267}