InstantExtended: record Instant NTP load times.

This uses the same histogram name as the existing NTP (Tab.NewTabOnload) which
should make it easy to see how it compares to the old load times.

BUG=325228
TESTED=manually, looking at about://histograms

Review URL: https://codereview.chromium.org/112453004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243188 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243188