Reland: Refactor ExtensionManifestTest to allow usage in src/extensions

Reland: Refactor ExtensionManifestTest to allow usage in src/extensions

Original CL broke the Win official bots because they report their default
version channel as "canary" instead of "trunk".

https://codereview.chromium.org/572813002

Original description:

This refactor will allow some manifest tests running in Chrome's unit_tests
suite to move to the extensions_unittests suite.

* Rename ExtensionManifestTest to extensions::ManifestTest and move it into
  src/extensions.
* Introduce ChromeManifestTest to load manifests from Chrome's test data dir.
* Eliminate some unnecessary use of chrome::VersionInfo::Channel.
* Move SharedModuleManifestTest to extensions_unittests as an example.

FileHandlerManifestTest and ExternallyConnectableManifestTest will be next,
but require more refactoring (e.g. of permissions).

BUG=397165
TEST=unit_tests, extensions_unittests
TBR=rockot@chromium.org

Committed: https://crrev.com/f79df3346ce523b2c44c7d99e0b819788c432203
Cr-Commit-Position: refs/heads/master@{#295225}

[James Cook, 2014-09-16 23:56:07]

jamescook@chromium.org changed reviewers:
+ rockot@chromium.org

[James Cook, 2014-09-16 23:56:21]

The CQ bit was checked by jamescook@chromium.org

[commit-bot: I haz the power, 2014-09-16 23:58:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/574173002/20001

[commit-bot: I haz the power, 2014-09-17 02:00:41]

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

[commit-bot: I haz the power, 2014-09-17 02:00:42]

Exceeded time limit waiting for builds to trigger.

[Ken Rockot(use gerrit already), 2014-09-17 03:43:15]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2014-09-17 03:43:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/574173002/20001

[commit-bot: I haz the power, 2014-09-17 04:05:19]

Committed patchset #2 (id:20001) as f77c1937a5ea3b999841f6657e9ee838ec61a13d

[commit-bot: I haz the power, 2014-09-17 04:05:52]

Patchset 2 (id:??) landed as
https://crrev.com/f79df3346ce523b2c44c7d99e0b819788c432203
Cr-Commit-Position: refs/heads/master@{#295225}