Disable double tap zoom on mobile sites, to remove 300ms click delay

This patch disables double tap zoom on pages that have a
width=device-width or narrower viewport (indicating that this is a
mobile-optimized or responsive website).

Double tap zoom continues to be disabled for pages that disallow the
user from zooming (even if they don't have a device-width or narrower
viewport).

This causes very noticeable performance benefits: links, buttons,
checkboxes, etc respond within around 10ms instead of 300ms.
Tested this on: http://jsbin.com/ixibol/6

Users shouldn't miss the double-tap gesture in the affected
situations, as double-tap means "zoom until tapped content is legible",
and in all the cases above the content was already legible (already
displayed at 1 or more DIPs per CSS pixel), so double tap zoom didn't
make sense and didn't do much.

BUG=169642
NOTRY=true

Review URL: https://codereview.chromium.org/18850005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232245 0039d316-1c4b-4281-b951-d872f2087c98
4 files changed