Fix phone-ar test in webxr-samples

This change is needed to match the API change in
https://chromium-review.googlesource.com/c/chromium/src/+/1570217
but was missing in that CL.

TBR=bajones@chromium.org

Change-Id: Ic49436558c3cde7e42b9b8a4bd26c416f0b80f2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574457
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Auto-Submit: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652689}

third_party/webxr_test_pages/webxr-samples/proposals/phone-ar.html

@@ -116,11 +116,11 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
         // activation event so that appropriate permissions can be granted.
         // This will likely prompt the user to allow camera use, so the promise
         // may remain outstanding for a while.
-        navigator.xr.requestSession({ mode: 'immersive-ar' }).then((session) => {
+        navigator.xr.requestSession('immersive-ar').then((session) => {
             xrButton.setSession(session);
             onSessionStarted(session);
         }).catch(() => {
-            navigator.xr.requestSession({ mode: 'legacy-inline-ar'})
+            navigator.xr.requestSession('legacy-inline-ar')
               .then((session) => {
                 xrButton.setSession(session);
                 onSessionStarted(session);