Add VR workaround for Incognito loading flakiness

Adds a couple of extra waitForTabPageLoaded to
VrBrowserNavigationTest#testIncognitoMaintainsSeparateHistoryStack as
a workaround for Incognito tabs reporting page load too quickly, which
was causing flakiness.

This can be removed once the root cause is fixed.

TBR=asimjour@chromium.org

Bug: 868506
Change-Id: Ie91a8213e641d9f51dc353d5215e8ac0695db059
Reviewed-on: https://chromium-review.googlesource.com/1153614Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578785}

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

@@ -568,8 +568,14 @@ public class VrBrowserNavigationTest {
         NewTabPageTestUtils.waitForNtpLoaded(mTestRule.getActivity().getActivityTab());
 
         // Test Incognito's forward/back.
+        // TODO(https://crbug.com/868506): Remove the waitForTabPageLoaded calls after the loadUrl
+        // calls once the issue with Incognito loadUrl reporting page load too quickly is fixed.
         mTestRule.loadUrl(TEST_PAGE_WEBVR_URL);
+        ChromeTabUtils.waitForTabPageLoaded(
+                mTestRule.getActivity().getActivityTab(), TEST_PAGE_WEBVR_URL);
         mTestRule.loadUrl(TEST_PAGE_WEBXR_URL);
+        ChromeTabUtils.waitForTabPageLoaded(
+                mTestRule.getActivity().getActivityTab(), TEST_PAGE_WEBXR_URL);
         VrBrowserTransitionUtils.navigateBack();
         ChromeTabUtils.waitForTabPageLoaded(
                 mTestRule.getActivity().getActivityTab(), TEST_PAGE_WEBVR_URL);