Revert of Revert of Move content:: scheme registrations from chrome/ to content/.

Move content:: scheme registrations from chrome/ to content/.

Both 'chrome:' and 'chrome-devtools:' are content-level schemes; we
should do their various renderer-side scheme registrations in content
(RenderThreadImpl::RegisterSchemes) rather than in chrome
(ChromeContentRendererClient::RenderThreadStarted).

Patch 1: https://codereview.chromium.org/1143593006
Patch 2: [This patch]
Patch 3: https://codereview.chromium.org/1139053004/

This is a re-land of https://codereview.chromium.org/1137103003/,
which was reverted due to trivial layout test changes.

TBR=jochen@chromium.org,philipj@opera.com,sofbjornf@opera.com,hiroshige@chromium.org
BUG=489672
Committed: https://crrev.com/8e94fb30db172bcc6bd4f22c84bf92ae6e18acb1
Cr-Commit-Position: refs/heads/master@{#330700}

[Mike West, 2015-05-19 19:07:53]

Created Revert of Revert of Move content:: scheme registrations from chrome/ to
content/.

[Mike West, 2015-05-19 19:09:54]

The CQ bit was checked by mkwst@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-19 19:10:58]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1134303009/1

[commit-bot: I haz the power, 2015-05-19 20:24:23]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-19 20:24:24]

Dry run: This issue passed the CQ dry run.

[Mike West, 2015-05-20 05:00:57]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2015-05-20 05:01:10]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1134303009/1

[commit-bot: I haz the power, 2015-05-20 05:05:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 05:06:21]

Patchset 1 (id:??) landed as
https://crrev.com/8e94fb30db172bcc6bd4f22c84bf92ae6e18acb1
Cr-Commit-Position: refs/heads/master@{#330700}