Work around VR controller disconnect flakiness

Adds a sleep (ew) after entering presentation in VR tests for testing
that the Daydream controller's app buton exits presentation. This works
around a VrCore issue when skipping the DON flow that causes the
controller to briefly disconnect and reconnect when entering VR, eating
any controller input that's sent during that time.

Bug: 870031
Change-Id: Id888826907c9548120d36b0dde67e30271d4bc85
Reviewed-on: https://chromium-review.googlesource.com/1159398Reviewed-by: Amirhossein Simjour <asimjour@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580225}

chrome/android/javatests/src/org/chromium/chrome/browser/vr/WebXrVrInputTest.java

@@ -386,6 +386,10 @@ public class WebXrVrInputTest {
             throws InterruptedException {
         framework.loadUrlAndAwaitInitialization(url, PAGE_LOAD_TIMEOUT_S);
         framework.enterSessionWithUserGestureOrFail();
+        // TODO(https://crbug.com/870031): Remove this sleep if/when the controller disconnect/
+        // reconnect issue caused by DON flow skipping that flakily eats the app button press is
+        // resolved.
+        SystemClock.sleep(500);
         EmulatedVrController controller = new EmulatedVrController(mTestRule.getActivity());
         controller.pressReleaseAppButton();
         assertAppButtonEffect(true /* shouldHaveExited */, framework);