Revert of Revert CL 117933003. Re-add resource speculative prefetching code.

Revert of Revert CL 117933003. Re-add resource speculative prefetching code. (patchset #10 id:180001 of https://codereview.chromium.org/462423004/)

Reason for revert:
This caused a bunch of memory leaks in following tests:

- ResourcePrefetcherTest.TestPrefetcherFinishes
- ResourcePrefetcherTest.TestPrefetcherStopped

Linux Chromium OS ASan LSan Tests (3)
http://build.chromium.org/p/chromium.memory/builders/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%283%29/builds/3216

Linux ASan LSan Tests (2)
http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%20LSan%20Tests%20%282%29/builds/7460

Original issue's description:
> Revert CL 117933003. Re-add speculative resource prefetching code.
> https://codereview.chromium.org/117933003/
>
> The speculative resource prefetching code was experimental code developed by
> shishir@. He found that it has little improvement on desktop Chrome (win). We
> though this should be beneficial to mobile browsers. After discussing with
> tburkard@ and kenjibaheux@, we decided to bring the code back and do more
> analysis on mobile devices. Reverting the patchset to re-add the code is the
> first step.
>
> The following design doc has discussed all related approaches and action items.
> https://docs.google.com/a/google.com/document/d/1ie3hu-zNNXvmTXm3aJAtKUGOh6nZfbNjA0aZE1bzzIg/edit?usp=sharing
>
> BUG=408399, 405690
>
> Committed: https://crrev.com/825722d5939dc0832d124b70958501611e6a4628
> Cr-Commit-Position: refs/heads/master@{#294899}

TBR=shishir@chromium.org,tburkard@chromium.org,kenjibaheux@chromium.org,jam@chromium.org,thestig@chromium.org,zhenw@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=408399, 405690
Committed: https://crrev.com/f03864238c8a90971f6b0c4d0f5acb894c0dfcd2
Cr-Commit-Position: refs/heads/master@{#295011}

[nhiroki, 2014-09-16 04:56:17]

Created Revert of Revert CL 117933003. Re-add resource speculative prefetching
code.

[commit-bot: I haz the power, 2014-09-16 04:56:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/577543002/1

[commit-bot: I haz the power, 2014-09-16 04:58:01]

Committed patchset #1 (id:1) as f6e502a0318533725f18d50d6e822beb9e8986ba

[commit-bot: I haz the power, 2014-09-16 04:59:23]

Patchset 1 (id:??) landed as
https://crrev.com/f03864238c8a90971f6b0c4d0f5acb894c0dfcd2
Cr-Commit-Position: refs/heads/master@{#295011}

[nhiroki, 2014-09-16 05:00:37]

This emitted a lot of memory leak errors and I'm not sure if this happened only
in tests, so let me revert...