Add pixel diff threshold to NewTabPageTest

Add a pixel diff threshold to NTP render tests to mitigate flakinesss on
Android CFI bot. Additional detail in linked bugs.

Bug: 945293, 1068294
Change-Id: Ia49455905dbb9f3ce2ff1809a73c8b0f30ff7f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137811Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756797}

chrome/android/javatests/src/org/chromium/chrome/browser/ntp/NewTabPageTest.java

@@ -185,6 +185,9 @@ public class NewTabPageTest {
         mFakebox = mNtp.getView().findViewById(R.id.search_box);
         mTileGridLayout = mNtp.getView().findViewById(R.id.tile_grid_layout);
         Assert.assertEquals(mSiteSuggestions.size(), mTileGridLayout.getChildCount());
+
+        // TODO(crbug.com/1068294): Remove the following line if this general issue is fixed.
+        mRenderTestRule.setPixelDiffThreshold(10);
     }
 
     @After