Previously, the PWindowGlobal actor was managed by the PBrowser which hosted it,
however this could cause issues when the tab containing the PWindowGlobal was
being destroyed. Due to the slightly different lifecycles of the PBrowser actor
and the nsGlobalWindowInner which the PWindowGlobal was trying to match the
lifetime of, the actor would sometimes not fire 'willDestroy' events correctly.

This patch moves PWindowGlobal to be directly managed by PContent, and changes
logic which previously used `Manager()` to get `Browser{Parent,Child}` to
instead use a pointer member.

Differential Revision: https://phabricator.services.mozilla.com/D68596

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extra : moz-landing-system : lando