Revert of scheduler: Implement task observers

Revert of scheduler: Implement task observers (patchset #7 id:120001 of https://codereview.chromium.org/922733002/)

Reason for revert:
Broke a CrOS test:

http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%28dbg%29%281%29/builds/37585/steps/browser_tests/logs/stdio

Original issue's description:
> scheduler: Implement task observers
>
> This patch adds a way for scheduler clients to get notified of task
> execution, much like with base::MessageLoop. This is needed to allow
> the execution of microtask checkpoints[1] between tasks by Blink while
> letting the scheduler use a >1 work batch size.
>
> This patch also refactors the various WebThreadImpls to reduce code
> duplication removes the deprecated postTask entrypoints that don't take
> the posting location as a parameter.
>
> BUG=450977, 444764
>
> [1] https://html.spec.whatwg.org/multipage/webappapis.html#processing-model-9
>
> Committed: https://crrev.com/ab211a0e344b431ca58c74f7815b15b28f7fb01a
> Cr-Commit-Position: refs/heads/master@{#316450}

TBR=alexclarke@chromium.org,rmcilroy@chromium.org,jam@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=450977, 444764
Committed: https://crrev.com/009f2cf40420e56e8259a6328ef3c265ff59ea27
Cr-Commit-Position: refs/heads/master@{#316479}

[Sami, 2015-02-16 14:41:29]

Created Revert of scheduler: Implement task observers

[commit-bot: I haz the power, 2015-02-16 14:42:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/930063002/1

[commit-bot: I haz the power, 2015-02-16 14:42:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-16 14:43:18]

Patchset 1 (id:??) landed as
https://crrev.com/009f2cf40420e56e8259a6328ef3c265ff59ea27
Cr-Commit-Position: refs/heads/master@{#316479}