Telemetry: Fix flakiness of Google calendar page in smoothness.top_25

For this page we execute a JS function to make the content scrollable. We then
immediately scroll. But sometimes the values we query for determining how far
to scroll haven't been updated. This patch introduces a short wait at the end of
navigate steps to ensure all values are up-to-date when we start scrolling.

R=ernstm@chromium.org, nduca@chromium.org
BUG=348434
NOTRY=True

Review URL: https://codereview.chromium.org/178473020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254869 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254869