Make CWS Report Abuse page the active tab after report abuse and uninstall

After a user clicks "Report Abuse" in the extension uninstall dialog, two
webpages may open -- the CWS Report Abuse page and an extension uninstall page,
if the extension specifies a custom uninstall page via
chrome.runtime.setUninstallURL.

Between these two webpages, the extension uninstall page becomes the in-focus,
active tab. Thus users will likely only fill out this form over the CWS Report
Abuse page.

This CL changes the active tab to become the CWS report abuse page instead of
the extension's uninstall page.

BUG=690050

Review-Url: https://codereview.chromium.org/2860663003
Cr-Commit-Position: refs/heads/master@{#473401}
5 files changed