Add a switch to enable/disable Instant search in about:flags.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/141703029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250652 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250652