Add a metric to check how often SiteInstances can reuse processes

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index 716da51d2b497e200be8727af58bb35fab5ad3a0..bbb68141231bdfc533f34aade0d62a12225298de 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -3103,6 +3103,9 @@ RenderProcessHost* RenderProcessHostImpl::GetProcessHostForSiteInstance(
     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;