Disable flaky testPrefetchPageReadyLater_FullBrowser on Tablet Testers

PrefetchFeedFlowTest#testPrefetchPageReadyLater_FullBrowser
is flaky on KitKat Tablet Tester bot.

BUG=1060685
TBR=carlosk@chromium.org,harringtond@chromium.org

Change-Id: I2e8cf3fac5d74d74c695b0bac474513e30e5d4bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2098522Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749277}

chrome/android/javatests/src/org/chromium/chrome/browser/offlinepages/prefetch/PrefetchFeedFlowTest.java

@@ -478,6 +478,7 @@ public class PrefetchFeedFlowTest {
     @Test
     @MediumTest
     @Feature({"OfflinePrefetchFeed"})
+    @DisableIf.Device(type = {UiDisableIf.TABLET}) // https://crbug.com/1060685
     public void testPrefetchPageReadyLater_FullBrowser() throws Throwable {
         doSetUp(/*isReducedMode=*/false);
         doTestPrefetchPageReadyLater();