Fix first run startup crash due to welcome ui

[simonhong]

This seems the regression from #130.
In that CL, BraveWelcomeUI WebUIController is introduced.
Because of this, current welcome ui page treated as webui.
This CL could be reverted when our local welcome ui is implemented.

close brave/brave-browser#315

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:

Reviewer Checklist:

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

[bbondy]

Since this is tracked in an issue and we will be converting to a local welcome page soon, merging.