Check for invalid URL during pushState flow.
Check for invalid URL during pushState flow.

Since there is no check in NavigationItem::SetURL for a
valid URL, the NavigationManager can get into a state
where it contains invalid URLs.

This CL adds a check for this case.

BUG=566214

Review URL: https://codereview.chromium.org/1546773002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367585}

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