Switch my_activity.py to use gerrit_util for gerrit REST connections

Switch my_activity.py to use gerrit_util for gerrit REST connections

Also added changes in gerrit_util.py to:
* Pick up the chrome-internal auth when chrome-internal-review is passed in,
  since in reality they share the same auth.
* Not use the /a/ endpoint if no auth is found in the netrc, so that
  chromium-reviews.googlesource.com will still work without a netrc auth.

BUG=281695

[Ryan Tseng, 2013-10-26 00:22:16]

+Szager: Can you take a look at this?

[deymo, 2013-10-30 21:36:33]

I'm fine with this change (although I don't really understand why are the
changes to gerrit_util.py needed). I just made a comment about removing most of
the contents of the function gerrit_changes_over_rest() since
gerrit_util.QueryChanges() already handles that part (with ReadHttpJsonResponse
+ CreateHttpConn).

Other than this, I don't see any conflict between this change and my proposed
change ( https://codereview.chromium.org/50283002/ ).

https://codereview.chromium.org/46413002/diff/1/my_activity.py
File my_activity.py (right):

https://codereview.chromium.org/46413002/diff/1/my_activity.py#newcode406
my_activity.py:406: def gerrit_changes_over_rest(instance, filters):
I think the same functionality implemented in this function can be achieved with
gerrit_util.QueryChanges() as I did in this patch:
https://codereview.chromium.org/50283002/diff/50001/my_activity.py

(the patch also includes the fix for bug 311649 using the QueryAllChanges()
function).

[Ryan Tseng, 2013-10-30 22:19:46]

Lets move to https://codereview.chromium.org/50283002/