Fix VR video pixel flakes

Fixes flakes in VrBrowserWebInputEditingTest#testFullscreenVideoControls
by allowing a 2 pixel threshold when comparing images. This is to work
around a random AA-related difference that popped up ~30% of the time.

TBR=billorr@chromium.org

Bug: 954993
Change-Id: Ia5ef23194c07c3955794034d9b1c3281b4e25b5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580299Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653448}

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

@@ -439,6 +439,9 @@ public class VrBrowserWebInputEditingTest {
     @Feature({"Browser", "RenderTest"})
     public void testFullscreenVideoControls()
             throws InterruptedException, TimeoutException, IOException {
+        // There's occasionally slight AA differences along the play button, so tolerate a small
+        // amount of differing.
+        mRenderTestRule.setPixelDiffThreshold(2);
         VrBrowserTransitionUtils.forceEnterVrBrowserOrFail(POLL_TIMEOUT_LONG_MS);
         NativeUiUtils.enableMockedInput();
         mVrBrowserTestFramework.loadUrlAndAwaitInitialization(