Relax pixel diff on Pie for ReturnToChromeTest#testInitialScrollIndex

The differences in web rendering can be ignored.

Bug: 1025241
Change-Id: I7f34e51ebdd22b0be97d35ccb9257bc8387ffa8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198640Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768880}

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

@@ -12,6 +12,8 @@ import static org.chromium.chrome.browser.tasks.ReturnToChromeExperimentsUtil.TA
 
 import android.app.Activity;
 import android.content.Intent;
+import android.os.Build;
+import android.os.Build.VERSION_CODES;
 import android.support.test.InstrumentationRegistry;
 import android.support.test.filters.MediumTest;
 import android.support.test.filters.SmallTest;
@@ -480,6 +482,10 @@ public class ReturnToChromeTest {
         assertEquals(9, mActivityTestRule.getActivity().getCurrentTabModel().index());
         // See crbug.com/1063619
         mRenderTestRule.setPixelDiffThreshold(2);
+        if (Build.VERSION.SDK_INT == VERSION_CODES.P) {
+            // See crbug.com/1025241
+            mRenderTestRule.setPixelDiffThreshold(16);
+        }
         // Make sure the grid tab switcher is scrolled down to show the selected tab.
         mRenderTestRule.render(mActivityTestRule.getActivity().findViewById(
                                        org.chromium.chrome.tab_ui.R.id.tab_list_view),