Revert of Implement timers by posting delayed tasks

Revert of Implement timers by posting delayed tasks (patchset #18 id:340001 of https://codereview.chromium.org/1134523002/)

Reason for revert:
The patch made some content_browsertests fail on Linux:
DomSerializerTests.SerializeHTMLDOMWithDocType
DomSerializerTests.SerializeHTMLDOMWithEmptyHead
DomSerializerTests.SerializeHTMLDOMWithEntitiesInText
DomSerializerTests.SerializeHTMLDOMWithMultipleMetaCharsetInOriginalDoc
DomSerializerTests.SerializeHTMLDOMWithNoMetaCharsetInOriginalDoc
DomSerializerTests.SerializeHTMLDOMWithoutDocType
RenderFrameHostManagerTest.DisownOpener

Original issue's description:
> Implement timers by posting delayed tasks
> 
> This patch refactors TimerBase to post tasks delayed tasks and
> deletes the now-obsolete timer heap and shared timer mechanism.
> 
> ATTN Sheriffs: If there are weird layout test flakes all of a
> sudden, this patch may be the cause since the interleaving of
> timers with other posted tasks will change.
> 
> Original patch by Alex Clarke <alexclarke@chromium.org>;.
> 
> BUG=463143, 416362, 480522
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195706

TBR=jochen@chromium.org,alexclarke@chromium.org,erikcorry@chromium.org,kinuko@chromium.org,rmcilroy@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=463143, 416362, 480522
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195725

[Julien - ping for review, 2015-05-21 19:59:44]

Created Revert of Implement timers by posting delayed tasks

[commit-bot: I haz the power, 2015-05-21 20:00:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1151633004/1

[commit-bot: I haz the power, 2015-05-21 20:01:09]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=195725