Revert of [Telemetry] Do not close tab connections if the tab is already gone.

Revert of [Telemetry] Do not close tab connections if the tab is already gone. (patchset #2 id:20001 of https://codereview.chromium.org/1041843002/)

Reason for revert:
https://crbug.com/471512
Flaky telemetry_perf_unittests on all platforms due to ChromeTracingStartedError

Setting current_tab to None should be done for the crash tab/app errors only.

Also, consider guarding the other clean-up methods that use it, or make sure they can dean with a None value.

Original issue's description:
> [Telemetry] Do not close tab connections if the tab is already gone.
> Update tests so they check for formatted exceptions (so they will catch
> a bug such as this one).
>
> BUG=444240
>
> Committed: https://crrev.com/86133f6e51b06f58afdcf4b00e65ffb3f510f34b
> Cr-Commit-Position: refs/heads/master@{#322613}

TBR=nednguyen@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=444240
Committed: https://crrev.com/ea14d2b3e757e609fd8fb4cdc580bdcfc591d8ae
Cr-Commit-Position: refs/heads/master@{#322738}

[slamm, 2015-03-30 04:00:05]

Created Revert of [Telemetry] Do not close tab connections if the tab is already
gone.

[commit-bot: I haz the power, 2015-03-30 04:00:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1047613002/1

[commit-bot: I haz the power, 2015-03-30 04:00:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-30 04:01:04]

Patchset 1 (id:??) landed as
https://crrev.com/ea14d2b3e757e609fd8fb4cdc580bdcfc591d8ae
Cr-Commit-Position: refs/heads/master@{#322738}