[Telemetry] Don't create a new Tab object at each navigation.

[Telemetry] Don't create a new Tab object at each navigation.

This patch prevents us from constructing a new Tab object at each navigation
which permits the tab contents video capture API to cache its tab contents
bounding box.

I also suspect it might help with flake and noise as we no longer connect and
disconnect so frequently and no longer depend on the python GC to do so.

BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=252246

[dtu, 2014-02-19 23:39:27]

This is a great cleanup. Hope it sticks.

https://chromiumcodereview.appspot.com/170283006/diff/40001/tools/telemetry/t...
File tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py (right):

https://chromiumcodereview.appspot.com/170283006/diff/40001/tools/telemetry/t...
tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py:20:
self._tab_dict = {}
Yay!

Hey, we didn't know about it at the time, but possibly TabListBackend should
extend collections.Sequence and ExtensionDictBackend should extend
collections.Mapping. Just something to think about!

[dtu, 2014-02-19 23:39:51]

On 2014/02/19 23:39:27, dtu wrote:
> This is a great cleanup. Hope it sticks.
> 
>
https://chromiumcodereview.appspot.com/170283006/diff/40001/tools/telemetry/t...
> File tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py
(right):
> 
>
https://chromiumcodereview.appspot.com/170283006/diff/40001/tools/telemetry/t...
> tools/telemetry/telemetry/core/backends/chrome/tab_list_backend.py:20:
> self._tab_dict = {}
> Yay!
> 
> Hey, we didn't know about it at the time, but possibly TabListBackend should
> extend collections.Sequence and ExtensionDictBackend should extend
> collections.Mapping. Just something to think about!

lgtm

[tonyg, 2014-02-19 23:49:06]

The CQ bit was checked by tonyg@chromium.org

[commit-bot: I haz the power, 2014-02-20 01:20:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tonyg@chromium.org/170283006/40001

[commit-bot: I haz the power, 2014-02-20 05:08:59]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tonyg@chromium.org/170283006/40001

[commit-bot: I haz the power, 2014-02-20 09:04:26]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tonyg@chromium.org/170283006/40001

[commit-bot: I haz the power, 2014-02-20 12:27:35]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/tonyg@chromium.org/170283006/40001

[commit-bot: I haz the power, 2014-02-20 15:54:22]

Change committed as 252246

[achuithb, 2014-02-20 20:02:35]

I suspect this CL broke ~12 telemetry unit tests on cros:
http://chromegw.corp.google.com/i/chromium.fyi/builders/Chromium%20Cros64%20P...

We really need a cros try server!!

(cros chrome bots are fine)