Revert r228800: "When a modal window appears, cancel existing touches."

TBR=tdresser@chromium.org
BUG=268075

Due to memory leak on chromiumos bot.

Review URL: https://codereview.chromium.org/26731003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228827 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228827