GitLab

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

Review URL: https://codereview.chromium.org/667843005

Cr-Commit-Position: refs/heads/master@{#301076}