Disabled pinch zooming in undocked devtools window.

BUG=348603

Review URL: https://codereview.chromium.org/176843010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255076 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255076