Stop reference counting BrowserContext

BrowserContext is currently reference counted, with WebContext holding a reference and WebViewContentsHelper holding a reference for each WebContents. We currently use base::RefCountedThreadSafe in order to customize the delete behaviour (BrowserContext is deleted via BrowserContextDestroyer, to ensure it outlives any RenderProcessHosts), but this is confusing because the class isn't thread safe (it also uses base::NonThreadSafe and we assert on most entry points that they are called on the UI thread).

We should stop reference counting it and have WebContext own it exclusively. BrowserContextDestroyer should then ensure it outlives any WebContents. Any classes that want to reference a BrowserContext should either implement BrowserContextObserver (which provides a notification when it's deleted) or use BrowserContextID (and we could provide a helper to map between BrowserContextID and BrowserContext)