Doug Thayer d61ea08ebb Bug 1665459 - Control pre-xul skele UI registry value via prefs r=mhowell
This patch uses the prefs relevant to the pre-xul skeleton UI to control the
registry value. There are three prefs:

- A new pref intended to be the global toggle once all themes can be handled.
- browser.startup.blankWindow - we currently depend on the code in BrowserGlue.jsm,
  gated on this pref, which immediately specifies the width and height of the
  window and ultimately causes XUL to consume and take ownership of our pre-xul
  window. Without this, we resize to fit a small content area, and then resize
  again back to the correct size.
- extensions.activeThemeID - Given that we have hardcoded layout and colors, we
  need to ensure that we're only presenting the skeleton UI for the default theme.

We're hoping to not need to gate on any prefs other than the new pref that we're
adding, but this allows us to provide a simple direction for users who may want
to dogfood our changes in the mean time: use the default theme, ensure the
browser.startup.blankWindow pref is set, and turn our new pref on.

Differential Revision: https://phabricator.services.mozilla.com/D90884
2020-09-21 19:24:56 +00:00
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