Add a metric to check how often SiteInstances can reuse processes

content/browser/renderer_host/render_process_host_impl.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Represents the browser side of the browser <--> renderer communication
 // channel. There will be one RenderProcessHost per renderer process.
 
 #include "content/browser/renderer_host/render_process_host_impl.h"
 
 #include <algorithm>
@@ skipping 3085 matching lines @@
       render_process_host = GetProcessHostForSite(browser_context, site_url);
       break;
     case SiteInstanceImpl::ProcessReusePolicy::USE_DEFAULT_SUBFRAME_PROCESS:
       DCHECK(SiteIsolationPolicy::IsTopDocumentIsolationEnabled());
       render_process_host = GetDefaultSubframeProcessHost(
           browser_context, site_instance, is_for_guests_only);
       break;
     case SiteInstanceImpl::ProcessReusePolicy::REUSE_PENDING_OR_COMMITTED_SITE:
       render_process_host =
           FindReusableProcessHostForSite(browser_context, site_url);
+      UMA_HISTOGRAM_BOOLEAN(
+          "SiteIsolation.ReusePendingOrCommittedSite.CouldReuse",
+          render_process_host != nullptr);
       break;
     default:
       break;
   }
 
   // If not (or if none found), see if we should reuse an existing process.
   if (!render_process_host &&
       ShouldTryToUseExistingProcessHost(browser_context, site_url)) {
     render_process_host = GetExistingProcessHost(browser_context, site_url);
   }
@@ skipping 510 matching lines @@
   LOG(ERROR) << "Terminating render process for bad Mojo message: " << error;
 
   // The ReceivedBadMessage call below will trigger a DumpWithoutCrashing.
   // Capture the error message in a crash key value.
   base::debug::ScopedCrashKey error_key_value("mojo-message-error", error);
   bad_message::ReceivedBadMessage(render_process_id,
                                   bad_message::RPH_MOJO_PROCESS_ERROR);
 }
 
 }  // namespace content