Add GetMimeTypeFromFile to Mojo's network service.

Add GetMimeTypeFromFile to Mojo's network service.

This allows loading file:// URLs as html (etc.) content.
This resolves https://codereview.chromium.org/1268993003 TODO.

It's also a compromise of the following requirements:
1) Use net/base/mime_util.* to infer MIME from file paths.
(we want a single robust and consistent inference of MIME types)
2) Do not move the mime_util code out of src/net.
(net can't depend on components; this code doesn't belong in base)
3) Avoid using the network service to load file URLs.
(we want to avoid file system access by the network service)

BUG=513779
TEST=Mojo and Mandoline handle html (etc.) file URLs as expected.
R=jam@chromium.org

Committed: https://crrev.com/669dda3d77f4ba264569fa37e7f12aa3fffd4df1
Cr-Commit-Position: refs/heads/master@{#341998}

[msw, 2015-08-04 20:41:53]

Hey John, please take a look; thanks!

[jam, 2015-08-04 23:10:08]

lgtm

[msw, 2015-08-04 23:32:13]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-08-04 23:34:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1274433003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1274433003/20001

[commit-bot: I haz the power, 2015-08-05 00:04:20]

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

[commit-bot: I haz the power, 2015-08-05 00:04:21]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[msw, 2015-08-05 18:09:05]

The CQ bit was checked by msw@chromium.org

[msw, 2015-08-05 18:09:06]

The patchset sent to the CQ was uploaded after l-g-t-m from jam@chromium.org
Link to the patchset: https://codereview.chromium.org/1274433003/#ps40001
(title: "Fix Windows compile error.")

[commit-bot: I haz the power, 2015-08-05 18:10:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1274433003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1274433003/40001

[commit-bot: I haz the power, 2015-08-05 19:06:50]

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

[commit-bot: I haz the power, 2015-08-05 19:06:50]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[msw, 2015-08-05 21:49:52]

Hey John, please take a look at the latest patch set; thanks!

[msw, 2015-08-05 21:50:16]

The CQ bit was checked by msw@chromium.org

[msw, 2015-08-05 21:50:17]

The patchset sent to the CQ was uploaded after l-g-t-m from jam@chromium.org
Link to the patchset: https://codereview.chromium.org/1274433003/#ps60001
(title: "Fix string usage and NativeApplicationLoaderTest.")

[commit-bot: I haz the power, 2015-08-05 21:51:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1274433003/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1274433003/60001

[jam, 2015-08-05 21:54:04]

lgtm

[commit-bot: I haz the power, 2015-08-05 22:55:15]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-08-05 22:55:54]

Patchset 4 (id:??) landed as
https://crrev.com/669dda3d77f4ba264569fa37e7f12aa3fffd4df1
Cr-Commit-Position: refs/heads/master@{#341998}