Disallow renderer crashes after RenderProcessHostWatcher::Wait().

After this CL renderer crashes won't be allowed after
RenderProcessHostWatcher::Wait returns.

Note that even before this CL, RenderProcessHostWatcher could only be
used once (i.e. for one process exit).  In particular,
|std::move(quit_closure_).Run()| would have hit a DCHECK when run a
second time.

Fixed: 1052257
Change-Id: Id1cbd9f59de57d5fe51131a5c7ac5ee14302d0d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068950
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743990}

content/public/test/browser_test_utils.cc

@@ -2386,6 +2386,10 @@ RenderProcessHostWatcher::~RenderProcessHostWatcher() {
 
 void RenderProcessHostWatcher::Wait() {
   run_loop_.Run();
+
+  DCHECK(allow_renderer_crashes_)
+      << "RenderProcessHostWatcher::Wait() may only be called once";
+  allow_renderer_crashes_.reset();
 }
 
 void RenderProcessHostWatcher::RenderProcessReady(RenderProcessHost* host) {