Revert of Move fetching logic out of ApplicationManager, eliminate url mappings.

Revert of Move fetching logic out of ApplicationManager, eliminate url mappings. (patchset #14 id:260001 of https://codereview.chromium.org/1342503003/ )

Reason for revert:
Broke the gn builds:
ERROR at //mojo/fetcher/about_fetcher_unittest.cc:17:11: Can't include this header from here.
#include "mojo/fetcher/about_fetcher.h"
          ^---------------------------
The target:
  //mojo/runner:mojo_runner_unittests
is including a file from the target:
  //mojo/fetcher:fetcher

It's usually best to depend directly on the destination target.
In some cases, the destination target is considered a subcomponent
of an intermediate target. In this case, the intermediate target
should depend publicly on the destination to forward the ability
to include headers.

Dependency chain (there may also be others):
  //mojo/runner:mojo_runner_unittests -->
  //mojo/runner:lib --[private]-->
  //mojo/fetcher:fetcher

https://build.chromium.org/p/chromium.linux/builders/Linux%20GN/builds/32017/steps/gn_check/logs/stdio

Original issue's description:
> Move fetching logic out of ApplicationManager, eliminate url mappings.
>
> BUG=
>
> Committed: https://crrev.com/7134773ceeef31531b84818158902e6d7c74f110
> Cr-Commit-Position: refs/heads/master@{#348659}
>
> Committed: https://crrev.com/c7ef4d2131963307ea537fc269bb5acee9383de2
> Cr-Commit-Position: refs/heads/master@{#348954}

TBR=sky@chromium.org,yzshen@chromium.org,ben@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Committed: https://crrev.com/295cbb2e343a6502334110c4d64189df6f2d83fa
Cr-Commit-Position: refs/heads/master@{#348979}

[Devlin, 2015-09-15 20:28:30]

Created Revert of Move fetching logic out of ApplicationManager, eliminate url
mappings.

[commit-bot: I haz the power, 2015-09-15 20:28:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1338283003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1338283003/1

[commit-bot: I haz the power, 2015-09-15 20:30:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 20:31:19]

Patchset 1 (id:??) landed as
https://crrev.com/295cbb2e343a6502334110c4d64189df6f2d83fa
Cr-Commit-Position: refs/heads/master@{#348979}

[commit-bot: I haz the power, 2015-09-23 12:48:58]

Patchset 1 (id:??) landed as
https://crrev.com/295cbb2e343a6502334110c4d64189df6f2d83fa
Cr-Commit-Position: refs/heads/master@{#348979}