webui: Stop propagation of ESC key event in dialog boxes.

Originally ESC key event to close the dialog box is propagated to the container
of dialog box and this causes an unexpected call of the container's handler.
This CL add a call of stopPropagation to prevent it.

BUG=282215
TEST=manually

Review URL: https://codereview.chromium.org/26458003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227720