Block javascript: document navigations during page dismissal events.

      This basically reflects the logic from FrameLoader::startLoad. Before this patch, javascript: document navigations could be performed during page dismissal events. This could be problematic, especially that dismissal events prevent loaders from being stopped or detached.

This patch adds a bail-out condition to FrameLoader::replaceDocumentWhileExecutingJavaScriptURL.

BUG=556724

Review URL: https://codereview.chromium.org/1451123002

Cr-Commit-Position: refs/heads/master@{#360242}