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from blank page (about:blank) that has been loaded in WebUI-bindings-enabled renderer.

This currently reproduces in process models other than default
process-per-site(-instance) as they only enforce new renderer
on webui-vs-regular navigations and fail to recognize that about:blank
might have been loaded in WebUI (as per http://crbug.com/42547).

This CL includes the browser test that reproduces this problem
and verifies the fix.

BUG=424526

Review URL: https://codereview.chromium.org/659293002

Cr-Commit-Position: refs/heads/master@{#300645}