Fix MojoApplicationHost assertion when navigating from RenderProcessGone.
Termination of a renderer process results in a call to
WebContentsObserver::RenderProcessGone. Navigating from
RenderProcessGone using NavigationControllerImpl::Reload or LoadURL
results in an assertion due to MojoApplicationHost::Init() being called
again on an already initialized host. Reset |mojo_application_host_| in
RenderProcessHostImpl::ProcessDied before calling
ViewHostMsg_RenderProcessGone message handlers to avoid this problem.
BUG=448435
Review URL: https://codereview.chromium.org/
848073002
Cr-Commit-Position: refs/heads/master@{#314435}