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Putative fix for crbug.com/545479. If WebUIManager is created but
navigation occurs before the WebUI URL starts loading, provisional
navigation for a non-WebUI URL could be observed by WebUIManager, and
WebUI should not be loaded in that case.

BUG=545479

Review URL: https://codereview.chromium.org/1464383002

Cr-Commit-Position: refs/heads/master@{#361143}