Revert of Revert of Abstract //content-level dependencies out of InMemoryURLIndex

Revert of Revert of Abstract //content-level dependencies out of InMemoryURLIndex (patchset #1 id:1 of https://codereview.chromium.org/1184923006/)

Reason for revert:
This revert broke other things. Reverting the revert.

Original issue's description:
> Revert of Abstract //content-level dependencies out of InMemoryURLIndex (patchset #5 id:100001 of https://codereview.chromium.org/1186103002/)
>
> Reason for revert:
> Speculative revert to fix the Windows unit_test failures.
>
> Original issue's description:
> > Abstract //content-level dependencies out of InMemoryURLIndex
> >
> > It now takes a base::SequencedWorkerPool and a set of client schemes to
> > whitelist in its constructor.
> >
> > BUG=371538
> >
> > TBR=phajdan.jr
> >
> > Committed: https://crrev.com/a6da40f8abbf44f8dab8db59a425b85c1db44376
> > Cr-Commit-Position: refs/heads/master@{#334799}
>
> TBR=pkasting@chromium.org,phajdan.jr@chromium.org,blundell@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=371538
>
> Committed: https://crrev.com/9d9c3c5655eff730578dc1e9524139ccd60fdb98
> Cr-Commit-Position: refs/heads/master@{#334866}

TBR=pkasting@chromium.org,phajdan.jr@chromium.org,blundell@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=371538
Committed: https://crrev.com/b834d85d0e2c3152a794d11fab93542cdc745c49
Cr-Commit-Position: refs/heads/master@{#334872}

[paulmeyer, 2015-06-17 18:02:10]

Created Revert of Revert of Abstract //content-level dependencies out of
InMemoryURLIndex

[commit-bot: I haz the power, 2015-06-17 18:02:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1181353003/1

[commit-bot: I haz the power, 2015-06-17 18:04:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-17 18:05:27]

Patchset 1 (id:??) landed as
https://crrev.com/b834d85d0e2c3152a794d11fab93542cdc745c49
Cr-Commit-Position: refs/heads/master@{#334872}