DOMUI: Two fixes for CC editor.

DOMUI: Two fixes for CC editor.
* Only show the obfuscated CC number on editor load.
* Clear the input field if the user modifies the obfuscated number.

BUG=70452
TEST=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=72577

[dhollowa, 2011-01-26 00:51:38]

LGTM.  Rubber stamp on JS style, but the logic looks good.

[Ilya Sherman, 2011-01-26 00:57:16]

What if I edit the number using only the mouse (select all, paste)?

[Ilya Sherman, 2011-01-26 00:59:08]

Also a behavioral bug: when adding a new credit card number, the first digit
typed is cleared as you type the second digit.

[Ilya Sherman, 2011-01-26 01:07:58]

More verbose:

Why do we want the obfuscated number to magically disappear when the user begins
editing the number?  This seems unnecessarily magical and confusing -- if I want
to change the number, I know how to select the text and delete it before doing
so.

Independently of that, I find the "hasEditedNumber_" logic odd.  Two
alternatives that make more sense to me:
(1) Store the obfuscated number as well as the non-obfuscated number.  Determine
whether the field has been edited by comparing to the stored obfuscated value.
(2) Consider the field to have been editing only if all of the characters are
digits.

If you'd like to keep the current logic, though, I guess oninput() or onchange()
would be more appropriate than onkeydown().