Revert of Revert "Post loading tasks on the appropriate WebFrameScheduler's queue."

Revert of Revert "Post loading tasks on the appropriate WebFrameScheduler's queue." (patchset #2 id:20001 of https://codereview.chromium.org/1402933002/ )

Reason for revert:
The failures happened again after reverting the original CL, so relanding that.

Original issue's description:
> Revert "Post loading tasks on the appropriate WebFrameScheduler's queue."
> and "Fix a UAF in the BackgroundHtmlParser's constructor"
>
> This reverts commit https://crrev.com/f3080498facd16066076e7d459ba7cfda11e582c and https://crrev.com/76d7303845116b1c08bd327c46c99e264b626ada.
> (CLs: https://codereview.chromium.org/1366883002 and https://codereview.chromium.org/1391013004.)
>
> Reason: Seems to have broken lots of browser_tests on WinXP.
>
> TBR=jochen@chromium.org
>
> BUG=510398, 538660, 541177
>
> Committed: https://crrev.com/57587e3c7053dc33118ba5a2603879fc6010f6de
> Cr-Commit-Position: refs/heads/master@{#353983}

TBR=alexclarke@chromium.org,jochen@chromium.org,skyostil@chromium.org,sandersd@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=510398, 538660, 541177
Committed: https://crrev.com/87d364f6ed284ae24a2c7ff7de41250e3a23f244
Cr-Commit-Position: refs/heads/master@{#354006}

[Marc Treib, 2015-10-14 12:39:12]

Created Revert of Revert "Post loading tasks on the appropriate
WebFrameScheduler's queue."

[commit-bot: I haz the power, 2015-10-14 12:40:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1402103002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1402103002/1

[commit-bot: I haz the power, 2015-10-14 12:42:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-14 12:43:03]

Patchset 1 (id:??) landed as
https://crrev.com/87d364f6ed284ae24a2c7ff7de41250e3a23f244
Cr-Commit-Position: refs/heads/master@{#354006}