Fixing use of uninitialized PLM UKM value

render_process_assignment_ was uninitialized and can be used in OnHidden
before OnCommit happens. We'll move to the optional pattern used in this
file.

Bug: 1157751
Change-Id: I4c126735ef51b126b93208cc61a1a08802bde802
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585991Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836206}

chrome/browser/page_load_metrics/observers/core/ukm_page_load_metrics_observer.cc

@@ -757,9 +757,6 @@ void UkmPageLoadMetricsObserver::RecordPageLoadMetrics(
         foreground_duration.value().InMilliseconds());
   }
 
-  builder.SetSiteInstanceRenderProcessAssignment(
-      SiteInstanceRenderProcessAssignmentToInt(render_process_assignment_));
-
   // Convert to the EffectiveConnectionType as used in SystemProfileProto
   // before persisting the metric.
   metrics::SystemProfileProto::Network::EffectiveConnectionType
@@ -803,8 +800,11 @@ void UkmPageLoadMetricsObserver::RecordPageLoadMetrics(
 void UkmPageLoadMetricsObserver::RecordRendererUsageMetrics() {
   ukm::builders::PageLoad builder(GetDelegate().GetPageUkmSourceId());
 
+  if (render_process_assignment_) {
     builder.SetSiteInstanceRenderProcessAssignment(
-      SiteInstanceRenderProcessAssignmentToInt(render_process_assignment_));
+        SiteInstanceRenderProcessAssignmentToInt(
+            render_process_assignment_.value()));
+  }
 
   builder.Record(ukm::UkmRecorder::Get());
 }

chrome/browser/page_load_metrics/observers/core/ukm_page_load_metrics_observer.h

@@ -222,7 +222,8 @@ class UkmPageLoadMetricsObserver
   base::Optional<net::LoadTimingInfo> main_frame_timing_;
 
   // How the SiteInstance for the committed page was assigned a renderer.
-  content::SiteInstanceProcessAssignment render_process_assignment_;
+  base::Optional<content::SiteInstanceProcessAssignment>
+      render_process_assignment_;
 
   // PAGE_TRANSITION_LINK is the default PageTransition value.
   ui::PageTransition page_transition_ = ui::PAGE_TRANSITION_LINK;