telemetry: Wait for messages to load before scrolling on GMail

If we try to scroll immediately after the message list div appears on
GMail, the messages may not have been loaded yet and something on the
page inhibits scrolling. Since there does not seem to be an obvious way
to detect when it is safe to scroll, just wait for 5 seconds before
continuing.

NOTRY=True
BUG=313523

Review URL: https://codereview.chromium.org/70353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234629 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234629