Make WebChannelIPCTransport into a RenderFrameObserver

As of version 63, Chromium creates proxy frames also for the main frame in the
frame tree during cross-process navigations. This leads to a segmentation fault
in WebChannelIPCTransport because we assume that all main frames are local.

See https://crrev.com/27caae83cb530daaf49f9a38793e427cdf493a65

 for details.

This patch refactors the renderer-side WebChannelIPCTransport from a
RenderViewObserver into a RenderFrameObserver, which prevents the segmentation
fault since the RenderFrameObserver is not created for proxy frames. Most likely
this would have to be done eventually anyway since the RenderView and
RenderViewObserver classes are deprecated and will likely be removed as part of
the Site Isolation project.

Installation is changed to follow Chromium's RenderFrameImpl in the sense of
performing the installation from RenderFrameObserver::DidClearWindowObject
instead of ContentRendererClient::RunScriptsAtDocumentStart. This has the
benefit of avoiding the ScriptForbiddenScope DCHECK.

Additionally there are the following minor changes:

  - The deprecated parameterless version of v8::Value::ToObject() method is
    replaced with v8::Value::IsObject() check and v8::Local::Cast.

  - The deprecated v8::Handle typedef is replaced with v8::Local.

  - The deprecated single-parameter WebContentsObserver::OnMessageReceived is
    replaced with the new two-parameter version.

  - blink::MainThreadIsolate() is used instead of v8::Isolate::GetCurrent() for
    Install/Uninstall since we know we are executing on the main thread.

  - WebChannelIPCTransportHost is changed to ignore messages from unexpected
    renderers in case something goes wrong with the renderers.

  - Logging is added to WebChannelIPCTransportHost for debugging purposes.

Some new unit tests are added, all of which fail with the old version.

Task-number: QTBUG-66333
Change-Id: I936d142fb042d9f936a3f9d08d4328ecba595f1f
Reviewed-by: Michal Klocek <michal.klocek@qt.io>