Basic view management for engine classes

[dkwingsmt]

This PR is part of the multiview engine project. For a complete roadmap, see this doc.

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This PR adds view management to all engine classes that need it. View management here basically means AddView and RemoveView methods, and most importantly, how to handle the implicit view.

The implicit view is a special view that's handled differently than all the other "regular views", since it keeps the behavior of the current single view of Flutter. Detailed introduction can be found in Settings.implicit_view_enabled.

The following two graphs show the difference between initializing with/without the implicit view and creating regular views.

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[dnfield]

Should these be DCHECK or CHECKs?

DCHECK will only run in a local unopt engine build. Flutter developers will never see these.

It seems like this should always be an error log at the least. Alternatively, it could be a DCHECK and an error log with a fizzle, e.g.

if (view_id == kFlutterImplicitViewId) {
  FML_LOG(ERROR) << "The implicit view #" << view_id << " cannot be removed.";
  FML_DCHECK(false);
  return;
}

[dnfield]

Similar comment for the other DCHECK on 95

[dkwingsmt]

I've changed one of them to the "if" pattern you suggested. I think both should stay DCHECK since they should never happen if Shell is implemented correctly, and serve as debug-only assertions. I changed the first check to an if to prevent the implicit view being deleted, but didn't for the second check so to make sure the message is always sent to the Dart VM

[dnfield]

Please cache the field lookup somewhere, see how that's done in PlatformConfiguration::DidCreateIsolate.

[dnfield]

This should probably get cached as well (The lookup of _updateWindowMetrics).

[dnfield]

Couple more issues but after that I think should be good to go

[dkwingsmt]

I've also removed Window, but merging it to PlatformConfiguration. We've always wanted to do this, not only because Window has only one actual property (view_configuration) but also that the name is obsolete. I found any ways to cache the two methods would be more verbose than just throwing the class away right now.

[dnfield]

nit: Consider moving this to window.h or platform_configuration.h.

[auto-submit]

auto label is removed for flutter/engine/42991, due to - The status or check suite Linux linux_android_aot_engine has failed. Please fix the issues identified (or deflake) before re-applying this label.