InstantExtended: update tests for Instant Extended (part 7).

Work around some of our unit and interactive UI tests don't work with the
cacheable NTP. This is a no-op right now but will be necessary once the
cacheable NTP is on by default. We should clean up these tests after flipping
the defaults.

Also disable a test that keeps flaking.

(Note that this is split off from http://crrev.com/24733003.)

BUG=297915
R=jered@chromium.org

Review URL: https://codereview.chromium.org/70073008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235261 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235261