Get PageLoadMetricsBrowserTest passing under msan

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
index bd43931446f71d19bd76b7e1b020774e849e76cb..bcaa73dabc9e9e0c7d658ae50cc5226235aab77f 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
@@ -146,7 +146,7 @@ class TimingUpdatedObserver : public content::BrowserMessageFilter {
   ~TimingUpdatedObserver() override {}
 
   std::unique_ptr<base::RunLoop> run_loop_;
-  int matching_fields_;  // A bitvector composed from ExpectedTimingFields.
+  int matching_fields_ = 0;  // A bitvector composed from ExpectedTimingFields.
   bool matched_timing_update_ = false;
   int match_document_write_block_reload_ = 0;
 };