Add a flag that disables the click delay.

Adds a flag at chrome://flags#disable-click-delay.  When enabled all taps are immediately turned into click events, instead of being delayed by 300ms.  This means that double tap to zoom can accidentally invoke links or other actions.  This is an experiment to help us better weigh the trade-offs here.

BUG=306269

Review URL: https://codereview.chromium.org/27641002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230183 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230183