Make JavaScript alert and confirm dialog boxes driveable by keyboard

Make JavaScript alert and confirm dialog boxes driveable by keyboard.

This regressed in r60785. An error during refactoring used
-[JavaScriptAppModalDialogHelper textField] to determine whether an accessory
view was added to a JavaScript dialog box, but this isn't a simple getter, and
was allays returning true.

BUG=58299
TEST=- javascript:alert("x") allows return keypress to dismiss dialog (OK);
       if FKA is on, allows space bar operation
     - javascript:confirm("x") allows return keypress to dismiss dialog (OK);
       if FKA is on, has focus on Cancel, is tabbable between Cancel and OK,
       and allows space bar operation;
     - javascript:prompt("x") focuses text field and allows return keypress to
       dismiss dialog (OK);
       if FKA is on, is tabbable between text field and buttons, and allows
       space bar operation on buttons
     - Note: FKA is Full Keyboard Access, from System Preferences:Keyboard

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=61847

[Ben Goodger (Google), 2010-10-07 20:27:55]

LGTM