telemetry: Re-enable old_inbox_app in key_silk_cases.

This page used to freeze chrome in repeated benchmark runs. The problem cannot
be reproduced on tip of tree. Re-enabling the page.

R=dominikg@chromium.org
BUG=335772

Review URL: https://codereview.chromium.org/152763004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250317 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250317