Description of Change

Fixes #20181.

When we removed our use of MainMenu.xib in this chromium roll commit, we inadvertently made it such that awakeFromNib defined in atom_application.mm was no longer called, and as such our custom handler for urls would no longer be registered and fired.

This fixes that issue by moving the url registration logic into a new method registerURLHandler() and ensuring that it's called in PreMainMessageLoopStartCommon(), at the same point in startup flow where this logic was previously being invoked.

cc @MarshallOfSound @jkleinsc @nornagon

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed an issue where the open-url event was not properly being fired on macOS.