Use the correct WebContents for WebXR metrics

Bug: 935758

Change-Id: Icb10d092a1e7c7849d89646fe5337e7b6efe4449
Reviewed-on: https://chromium-review.googlesource.com/c/1487810
Commit-Queue: Suman Kancherla <sumankancherla@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635600}

chrome/browser/vr/ui_host/vr_ui_host_impl.cc

@@ -86,8 +86,7 @@ void VRUiHostImpl::SetWebXRWebContents(content::WebContents* contents) {
       metrics_helper->SetVRActive(false);
     }
     if (contents) {
-      auto* metrics_helper =
-          SessionMetricsHelper::FromWebContents(web_contents_);
+      auto* metrics_helper = SessionMetricsHelper::FromWebContents(contents);
       if (!metrics_helper) {
         metrics_helper = SessionMetricsHelper::CreateForWebContents(
             contents, Mode::kWebXrVrPresentation);