Make DocumentTabModelImpl fire didSelectTab

Make DocumentTabModelImpl fire didSelectTab

DocumentTabModelImpl now fires didSelectTab to its observers. This is done in
setLastShownId() instead of setIndex() for a few reasons:

* setIndex() has the side effect of bringing Tabs forward, which you may not
  want.

* setLastShownId() hopefully more faithfully tracks what Tab was last shown in
  the foreground, and is called by DocumentActivity.onResume().  This might
  still be incorrect (e.g. when opening a Tab in the background), but this is
  one of those weird cases where Android has two Activities in the foreground
  simultaneously.

* It's really difficult to actually indicate what brings a Tab back to the
  foreground.  Slap a TODO on there to actually fix it when an Observer actually
  needs to know the type.

BUG=443772
Committed: https://crrev.com/b265913295783b977eb6aa7294ebcb938cd8bf62
Cr-Commit-Position: refs/heads/master@{#322519}

[gone, 2015-03-27 00:23:21]

dfalcantara@chromium.org changed reviewers:
+ dtrainor@chromium.org, tedchoc@chromium.org

[Ted C, 2015-03-27 00:30:25]

lgtm

[gone, 2015-03-27 01:29:42]

The CQ bit was checked by dfalcantara@chromium.org

[commit-bot: I haz the power, 2015-03-27 01:30:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1039513003/1

[commit-bot: I haz the power, 2015-03-27 01:55:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-27 01:56:43]

Patchset 1 (id:??) landed as
https://crrev.com/b265913295783b977eb6aa7294ebcb938cd8bf62
Cr-Commit-Position: refs/heads/master@{#322519}