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WebUIs were not properly being created when reusing the current
RenderFrameHost. But while doing that I found an issue in
RenderFrameHostManager::CreateSpeculativeRenderFrameHost where instead
of setting an instance member I was mistakenly shadowing it with a
same-named function member.

This last fix caused another test to crash because the WebUI reuse logic
was incorrect. Instead of checking if it should be reused when the
navigation SiteInstance matched the current one, it was being done when
they were different (what doesn't even makes sense).

Due to these changes I also had to update how RFHM::CleanUpNavigation
and UnsetSpeculativeRenderFrameHost work,so that the speculative WebUI
member is always cleaned up even if a speculative RFH does not exist.

Also renamed the speculative WebUI getter as it's not test-only anymore.

With these changes these tests were fixed when PlzNavigate is enabled:
    RenderFrameHostManagerTest.EnableWebUIWithSwappedOutOpener
    RenderFrameHostManagerTest.WebUIInNewTab

BUG=439423

Review URL: https://codereview.chromium.org/877343010

Cr-Commit-Position: refs/heads/master@{#315006}