Disable ReturnToChromeTest#testInitialScrollIndex.

Fails consistently on Lollipop Phone Tester with TIMEOUT:
see e.g. https://ci.chromium.org/p/chromium/builders/ci/Lollipop%20Phone%20Tester/26466

Bug: 1063984
Tbr: yusufo@chromium.org
Change-Id: Icedf0752d949f6d3ca3a5c08667e2fe20a023e7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343546
Commit-Queue: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796108}

chrome/android/javatests/src/org/chromium/chrome/browser/tasks/ReturnToChromeTest.java

@@ -551,6 +551,7 @@ public class ReturnToChromeTest {
     @CommandLineFlags.Add({BASE_PARAMS + "/" + TAB_SWITCHER_ON_RETURN_MS_PARAM + "/0"
             + "/start_surface_variation/omniboxonly"})
     @Restriction(UiRestriction.RESTRICTION_TYPE_PHONE)
+    @DisabledTest(message = "https://crbug.com/1063984")
     public void testInitialScrollIndex() throws Exception {
         // clang-format on
         // Instant start is not applicable since we need to create tabs and restart.