Fix navigations in OOP PDF

Navigations broke as a result of being inside a browser plugin because the
embedded WebContents would be navigated instead of the top-level one.

This change switches to use the chrome.tabs API to do navigations which
fixes this problem.

BUG=446390

Review URL: https://codereview.chromium.org/787223005

Cr-Commit-Position: refs/heads/master@{#310062}
1 file changed