Prevent access to BrowserContextDelegate off the IO thread

[Chris Coulson]

Currently it's possible to call BrowserContext::GetDelegate() on any thread, but BrowserContextDelegate has methods whose implementations in qt/ are just plain unsafe to call on anything other than the IO thread (it looks like this applies to pretty much all of them).

I noticed this whilst looking at https://code.launchpad.net/~justinmcp/oxide/pepper-flash/+merge/234058 - it calls BrowserContextDelegate::CanAccessStorage on the UI thread, which is not safe.

We should prevent usage of BrowserContextDelegate off the IO thread.

For an alternative to BrowserContextDelegate::CanAccessStorage on the UI thread - this will depend on content settings as opposed to delegating storage access checks to an embedder provided worker script on the IO thread.

As an interim, we should make BrowserContextIOData::CanAccessCookies not delegate to BrowserContextDelegate - this is sufficient for checking whether a particular storage access attempt is ok, according to the current cookie policy. However, it won't be possible to have more fine-grained storage access checks off the IO thread until we have content settings.