[Chromcast] Call CastWebContentImpl::LoadUrl() instead of WebContent::GetController().LoadUrl()

CastWebContentImpl::LoadUrl() is never called when the webview instance
is CastWebViewDefault so that some initializations in
CastWebContentImpl::LoadUrl() would never be executed. For example, the
variable `start_loading_ticks_` cannot be setted-up correctly and the
loading time we got in CastWebContentImpl::DidStopLoading() would be
wrong. So, we should let CastWebViewDefault::LoadUrl() call
CastWebContentImpl::LoadUrl() instead of
WebContent::GetController().LoadUrl().

Bug=internal b/123603638
Test=build

Change-Id: I6d99bd4674730a6942b34b0140e08846f30fe0e3
Reviewed-on: https://chromium-review.googlesource.com/c/1445043
Commit-Queue: Chih-Hsuan Kuo <chkuo@google.com>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631537}