fix(chrome): a failed metrics round trip is an error, not zero bytes

performance.memory is absent on plenty of pages and zero is the honest answer
there. Swallowing the evaluate error gave that same answer when the round trip
never happened, so a run recorded a memory reading it never took.

internal/driver/chrome/driver.go

@@ -971,6 +971,10 @@ func (d *Driver) Health(_ context.Context) (driver.Health, error) {
 	}
 }
 
+// Metrics reports zero with a nil error on a page that does not expose
+// performance.memory, so a failed round trip has to be an error: the two are
+// otherwise the same answer, and a run would record a memory reading it never
+// took.
 func (d *Driver) Metrics(ctx context.Context, _ string) (driver.Metrics, error) {
 	runCtx, cancel := d.runCtx(ctx)
 	defer cancel()
@@ -981,7 +985,7 @@ func (d *Driver) Metrics(ctx context.Context, _ string) (driver.Metrics, error)
   return {heap: mem.usedJSHeapSize || 0, totalMem: mem.totalJSHeapSize || 0};
 })()`
 	if err := chromedp.Run(runCtx, chromedp.Evaluate(script, &result)); err != nil {
-		return driver.Metrics{}, nil
+		return driver.Metrics{}, fmt.Errorf("read performance.memory: %w", err)
 	}
 	heap, _ := result["heap"].(float64)
 	total, _ := result["totalMem"].(float64)

internal/driver/chrome/driver_test.go

@@ -1686,3 +1686,34 @@ func TestInstallBundle_AcceptsAPageThatInstallsNoPicker(t *testing.T) {
 		t.Fatalf("InstallBundle refused a bundle that generates no actions: %v", err)
 	}
 }
+
+// TestMetrics_AFailedRoundTripIsNotAPageWithoutTheAPI pins the distinction the
+// zero-and-nil return erased. performance.memory is absent on plenty of pages
+// and zero is the honest answer there, so a round trip that never happened has
+// to be an error or a run records a memory reading it never took.
+func TestMetrics_AFailedRoundTripIsNotAPageWithoutTheAPI(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(`<body><div id="app">app</div></body>`))
+	}))
+	defer server.Close()
+
+	d := New()
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL, false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+	if _, err := d.Metrics(ctx, ""); err != nil {
+		t.Fatalf("Metrics on a live page: %v", err)
+	}
+
+	if err := d.Terminate(context.Background()); err != nil {
+		t.Fatalf("Terminate: %v", err)
+	}
+	metrics, err := d.Metrics(ctx, "")
+	if err == nil {
+		t.Errorf("Metrics returned %+v and a nil error after the tab was gone; "+
+			"the run cannot tell that from a page with no performance.memory", metrics)
+	}
+}