Omnibox: Launch Stop Timer Behavior

Previously, we ran an experiment that caused all autocomplete providers
to stop delivering results after 1.5 seconds.  This was caused by the
assumption that results coming in after that length of time are
disruptive, likely causing more harm than benefit.  We decided to launch
this experiment.  This change turns on that behavior for everyone.

This change also converts the old "use stop timer" field trial code to
a parameterized form, so we can run future experiments with shorter
timeouts.

BUG=

Review URL: https://codereview.chromium.org/117233009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242915 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242915
Track v8 API changes.

R=jochen@chromium.org, dcarney@chromium.org
BUG=324225

Review URL: https://codereview.chromium.org/122073003

Patch from Sven Panne <svenpanne@chromium.org>.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242916 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242916