Add DCHECKs to validate ProfileIOData's list of protocols.

Add DCHECKs to validate ProfileIOData's list of protocols.

Every ProtocolHandler we create needs to be registered in ProfileIOData's
static list of built-in protocols.

Rename logic in a few places, using the term 'built-in' instead of
'handled', to distinguish between custom protocol handlers, and
real protocol handlers that might return a network response.

BUG=659492

[ncarter (slow), 2016-10-28 22:16:19]

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

[commit-bot: I haz the power, 2016-10-28 22:16:44]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-28 22:19:44]

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

[commit-bot: I haz the power, 2016-10-28 22:19:45]

Dry run: Try jobs failed on following builders:
  linux_chromium_asan_rel_ng on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  mac_chromium_compile_dbg_ng on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_rel_ng on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[ncarter (slow), 2016-10-28 22:31:08]

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

[commit-bot: I haz the power, 2016-10-28 22:32:09]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-28 22:50:24]

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

[commit-bot: I haz the power, 2016-10-28 22:50:25]

Dry run: Try jobs failed on following builders:
  android_n5x_swarming_rel on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_n5x_...)

[ncarter (slow), 2016-11-02 19:50:52]

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

[commit-bot: I haz the power, 2016-11-02 19:51:22]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-02 20:07:58]

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

[commit-bot: I haz the power, 2016-11-02 20:07:59]

Dry run: Try jobs failed on following builders:
  cast_shell_linux on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/cast_shell_linu...)