Fix release mode brave_unit_tests crash on Windows

[simonhong]

This is regression of #158.
During the initialization of unit test, channel info is accessed.
However, |g_module_details| isn't initialized in test.
Before the above PR, channel was always Channel::UNKNOWN.
So, channel info can be fetched without accessing |g_module_details|.
After the above PR, channel info is fetched via |g_module_details| in
release mode. So crash is happened because |g_module_details| is
nullptr.
Fixed by initializing install details explicitly.

Close: brave/brave-browser#443

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.

Test Plan:

Run yarn test brave_unit_tests Release on Windows

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions