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- when navigating, add host bindings to the pending frame rather than old frame;
- force renderer swap if front-end URL is invalid;
- move front-end URL validation to DevToolsUIBindingds

This also re-lands https://codereview.chromium.org/2607833002

BUG=662859,678035

Review-Url: https://codereview.chromium.org/2620153002
Cr-Commit-Position: refs/heads/master@{#442781}