Simplify content_layer attachment and viewporting logic.

Simplify content_layer attachment and viewporting logic.

I noticed a lot of complex logic in content_layer.cc and
static_tab_scene_layer.cc that appears to do nothing useful anymore.  I
simplified it by:
1) Always detaching and reattaching the live and static sublayers
instead of trying to preserve the existing structure.  (There shouldn't
be any performance cost to this.)
2) Following a much simpler policy for positioning and clipping.  I
compared the behavior of the old and new logic in a variety of device
rotation, tab switcher and URL-bar-hiding scenarios, and I haven't
yet observed any regression.

BUG=662427

[aelias_dont_use, 2016-11-12 23:10:43]

The CQ bit was checked by aelias@google.com to run a CQ dry run

[commit-bot: I haz the power, 2016-11-12 23:10:45]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[aelias_dont_use, 2016-11-12 23:25:58]

The CQ bit was checked by aelias@google.com to run a CQ dry run

[commit-bot: I haz the power, 2016-11-12 23:26:06]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[aelias_OOO_until_Jul13, 2016-11-12 23:30:55]

aelias@chromium.org changed reviewers:
+ changwan@chromium.org

[aelias_OOO_until_Jul13, 2016-11-12 23:31:03]

aelias@chromium.org changed reviewers:
+ aelias@chromium.org

[aelias_OOO_until_Jul13, 2016-11-12 23:31:03]

Hi changwan@, PTAL.

[commit-bot: I haz the power, 2016-11-13 00:09:43]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-13 00:09:44]

Dry run: Try jobs failed on following builders:
  android_clang_dbg_recipe on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_clan...)

[aelias_dont_use, 2016-11-13 01:13:34]

The CQ bit was checked by aelias@google.com to run a CQ dry run

[commit-bot: I haz the power, 2016-11-13 01:13:46]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-13 02:23:21]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-13 02:23:21]

Dry run: This issue passed the CQ dry run.

[aelias_OOO_until_Jul13, 2016-11-13 04:24:30]

I found a tablet-specific bug with this, so please hold off review for now.

[aelias_OOO_until_Jul13, 2016-12-09 23:47:03]

Closing this issue because I accidentally created it as aelias@google.com,
created another issue at https://codereview.chromium.org/2566753004