Ensure that navigation will proceed when no NavigationHandle is found

This is a fix for an issue where the NavigationHandle is destroyed by
the commit of a same-page navigation, which will cancel an ongoing
navigation. This patch allows the navigation to go through, and is meant
to be a temporary fix until https://codereview.chromium.org/2475693002/
lands.

BUG=658530

Review-Url: https://codereview.chromium.org/2481173002
Cr-Commit-Position: refs/heads/master@{#430650}
1 file changed