Fix tabbing after activating a delete button with focusing, then hitting the Enter key.

Previously, after hitting an Enter key after focusing any of commands buttons, the command was executed twice. This was happening, because two events were executing the same command - click and keypress. The second one is redundant, since hitting an Enter key invokes the click handler.

This behaviour caused showing the delete confirmation dialog twice. As a result, the tab prevention logic in dialogs.js (webui) was setting tabIndex to -1 on all focusable elements. In case of displaying the dialog two times in a row, also the dialog's buttons were set to not focusable. Since none of controls became focusable, the body became focused, which caused stealing the focus out of the dialog.

TEST=Tested manually.
BUG=325131

Review URL: https://codereview.chromium.org/103603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238682 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238682