Adds InstantExtended.CacheableNTPLoad histogram to histograms.xml

BUG=NONE

Review URL: https://codereview.chromium.org/26605002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227803 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227803