DevTools: fix race when revealing formatted source code

The porblem was caused by trying to format and add a source code twice.
As a drive by, do not reveal the source code if the user navigated away.

BUG=723098

Review-Url: https://codereview.chromium.org/2888843002
Cr-Commit-Position: refs/heads/master@{#473510}
3 files changed