Refactoring in the scanner and removing broken Domado features.

These changes are to support upcoming improvements to the scanner.

Domado:
* Removed HTMLMediaElement.prototype.fastSeek, as no browser implements
  it yet.
* window.pageXOffset and window.pageYOffset no longer have (broken)
  setters.

es53-test-scan-guest.js:
* noteGap takes an (optional) context arg like noteProblem.
* Factored out specialized whole-object checks from Scanner.
* annotate() will compose with an existing annotation rather than
  overwriting it silently.
* freshResult() will not fail if the result is not an object.
* Fixed testScanner passing an object instead of a string to
  scanner.skip().

@r5494

[kpreid2, 2013-07-15 20:47:49]

ping

[ihab.awad, 2013-07-15 21:51:51]

Not familiar with architecture of scanner so abandoning effort to review that
portion. Domado portion lgtm.

[ihab.awad, 2013-07-15 22:35:00]

To clarify:

* Domado portion lgtm;

* Scanner portion lgtm as-is since we have not shared information about how our
scanner is implemented (it was originally checked in with very light review)
and, though the scanner is very useful, disseminating knowledge about it is not
our group's immediate priority.

[felix8a, 2013-07-15 23:22:32]

I reviewed the initial checkin of the scanner. lgtm

https://codereview.appspot.com/11140043/diff/1/tests/com/google/caja/plugin/e...
File tests/com/google/caja/plugin/es53-test-scan-guest.js (right):

https://codereview.appspot.com/11140043/diff/1/tests/com/google/caja/plugin/e...
tests/com/google/caja/plugin/es53-test-scan-guest.js:1034: var original =
call[2];
putting the var here is odd, it took me a minute to connect it with the function
that used it and figure out the intent.