Enable pinch-to-zoom on CrOS by default.

Enabled old (Android-style) pinch-to-zoom by default in CrOS to match
Windows 8 change in r247390. Also made it so that it can be disabled
with --disable-pinch on both CrOS and Win8.

BUG=338498

Review URL: https://codereview.chromium.org/154403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249124 0039d316-1c4b-4281-b951-d872f2087c98
Handle null scroll layer in LayerTreeHostImpl::MouseMoveAt().

It is possible that LayerById() can return a NULL pointer if the id
passed in has become stale. The id can potentially become stale if
the layer tree is restructured while the mouse is over the scrollbar
layer.

BUG=326635

Review URL: https://codereview.chromium.org/155373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249125 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249125