Files.app: Refactoring for the onMessage function of FIleManagerBrowserTest.

Files.app: Refactoring for the onMessage function of FileManagerBrowserTest.

Previously onMessage function of FileManagerBrowserTest returns a string, but
the format of the return values is not consistent among the message types.

Also the onMessage function retunrs the value even if it is fatal error, but for
the case, we can stop the test directly in the function.

The CL removes unrequired result values, and adds test assertions for fatal
errors.

BUG=None
TEST=None

Committed: https://crrev.com/15161ce4e44d4945e6e7bdef17b2ca2d316e1578
Cr-Commit-Position: refs/heads/master@{#301076}

[hirono, 2014-10-23 06:55:22]

hirono@chromium.org changed reviewers:
+ mtomasz@chromium.org

[hirono, 2014-10-23 06:55:22]

PTAL the CL? Thank you!

[mtomasz, 2014-10-23 07:49:15]

lgtm!

[hirono, 2014-10-23 07:50:21]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-10-23 07:51:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/667843005/40001

[commit-bot: I haz the power, 2014-10-23 08:18:45]

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

[commit-bot: I haz the power, 2014-10-23 08:18:46]

Try jobs failed on following builders:
  linux_chromium_compile_dbg_32 on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[hirono, 2014-10-23 09:25:48]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-10-23 09:26:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/667843005/40001

[commit-bot: I haz the power, 2014-10-23 09:54:02]

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

[commit-bot: I haz the power, 2014-10-23 09:54:03]

Try jobs failed on following builders:
  linux_chromium_compile_dbg_32 on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  linux_chromium_rel on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[hirono, 2014-10-23 11:45:40]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-10-23 11:46:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/667843005/60001

[commit-bot: I haz the power, 2014-10-23 13:03:09]

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

[commit-bot: I haz the power, 2014-10-23 13:03:10]

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

[hirono, 2014-10-24 07:22:14]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-10-24 07:22:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/667843005/80001

[hirono, 2014-10-24 07:25:09]

The CQ bit was unchecked by hirono@chromium.org

[hirono, 2014-10-24 08:27:51]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-10-24 08:29:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/667843005/120001

[commit-bot: I haz the power, 2014-10-24 09:23:30]

Committed patchset #7 (id:120001)

[commit-bot: I haz the power, 2014-10-24 09:24:28]

Patchset 7 (id:??) landed as
https://crrev.com/15161ce4e44d4945e6e7bdef17b2ca2d316e1578
Cr-Commit-Position: refs/heads/master@{#301076}