Content Browser 1 Inconsistent Behavior with Tabs

"Content Browser 1" auto-hides if opened with the Level Editor, and has inconsistent docking behavior under the conditions caused by the repro steps above.

The root of the problem seems to be that whenever we open a Content Browser, `FTabManager` calls `InvokeTab_Internal()`. In the problem case, we invoke the Level Editor's tab manager, which only registers the first Content Browser (`ContentBrowserTab1`). Thus, Content Browser 1 is found in its tab manager under its `CollapsedDockAreas`. By contrast, `ContentBrowser2`, `ContentBrowser3`, etc. aren't in the Level Editor's tab manager's `CollapsedDockAreas` member variable, thereby causing `InvokeTab_Internal` to make a recursive call on the global tab manager. 

This problem does not occur when one opens any Content Browser from any non-Level-Editor tab. Additionally, Content Browsers 2, 3, and 4 don't suffer from this problem (for the Level Editor).

1. Open the Level Editor.
2. Open another window, and add it as a second tab to the Level Editor window. E.g. open a simple Cube in the Static Mesh Editor, and drag it over to the Level Editor as a second tab.
3. Go to the Level Editor tab. Open "Content Browser 1" (Windows -> Content Browser -> Content Browser 1).
4. Change content browser to its own freestanding window.
5. Now when you switch tabs, e.g. to the Static Mesh Editor, this content browser is automatically hidden/closed. If you attempt to open it from this tab (Windows -> Content Browser -> Content Browser 1), then you'll be brought back to the Level Editor