Revert of Explicitly track the scheduler task enqueueing order in a new field

Revert of Explicitly track the scheduler task enqueueing order in a new field (patchset #6 id:100001 of https://codereview.chromium.org/1259583006/ )

Reason for revert:
This is causing failures on a bunch of bots, particularly for the TaskQueueSelectorTest.TestAge test:
http://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests%20%28dbg%29/builds/9536/steps/components_unittests%20on%20Mac-10.9/logs/TaskQueueSelectorTest.TestAge
http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29/builds/45529/steps/components_unittests/logs/TaskQueueSelectorTest.TestAge
http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/17574/steps/components_unittests/logs/TaskQueueSelectorTest.TestAge

Also
BrowserCloseManagerBrowserTest_BrowserCloseManagerBrowserTest.PRE_TestSessionRestore_1
BrowserCloseManagerBrowserTest_BrowserCloseManagerBrowserTest.TestSessionRestore_1 failed on Mac in the same run. Not sure if related:
http://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests%20%28dbg%29/builds/9536/steps/browser_tests%20on%20Mac-10.9/logs/stdio

Original issue's description:
> Explicitly track the scheduler task enqueueing order in a new field
>
> Task enqueueing_order for delayed tasks is not known until the task's delay has
> expired.  Preciously we were storing this in sequence_num but that
> broke FlowEvents because the computed IDs changed mid-flight.
>
> In addition this patch no longer zeros out delayed_run_time
> since that appears to be used by some tracing code.
>
> BUG=515720
>
> Committed: https://crrev.com/8c6b40b808787f00fef268e6c06eb02a0f8f690e
> Cr-Commit-Position: refs/heads/master@{#342113}

TBR=skyostil@chromium.org,alexclarke@google.com,alexclarke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=515720
Committed: https://crrev.com/0902834d4c3130c9ba6f8c9dc337252532f2589b
Cr-Commit-Position: refs/heads/master@{#342116}

[mfomitchev, 2015-08-06 15:34:06]

Created Revert of Explicitly track the scheduler task enqueueing order in a new
field

[commit-bot: I haz the power, 2015-08-06 15:34:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1277923002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1277923002/1

[commit-bot: I haz the power, 2015-08-06 15:35:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-06 15:36:07]

Patchset 1 (id:??) landed as
https://crrev.com/0902834d4c3130c9ba6f8c9dc337252532f2589b
Cr-Commit-Position: refs/heads/master@{#342116}