Fix nullptr deref when Unaliasing an effect node.

My previous patch missed one spot where we can unalias a node which
is actually null. This patch fixes up that spot.

R=pdr@chromium.org, chrishtr@chromium.org

Bug: 884081, 884083, 884084, 884186
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic12a21c429c4984cb18f92f6beac8c18c0f51f87
Reviewed-on: https://chromium-review.googlesource.com/1227147Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591444}

third_party/blink/renderer/platform/graphics/compositing/chunk_to_layer_mapper.cc

@@ -37,7 +37,7 @@ void ChunkToLayerMapper::SwitchToChunk(const PaintChunk& chunk) {
     new_has_filter_that_moves_pixels = false;
     for (const auto* effect = new_chunk_state.Effect();
          effect && effect != layer_state_.Effect();
-         effect = effect->Parent()->Unalias()) {
+         effect = effect->Parent() ? effect->Parent()->Unalias() : nullptr) {
       if (effect->HasFilterThatMovesPixels()) {
         new_has_filter_that_moves_pixels = true;
         break;

third_party/blink/renderer/platform/graphics/compositing/chunk_to_layer_mapper_test.cc

@@ -224,4 +224,18 @@ TEST_F(ChunkToLayerMapperTest, SlowPath) {
   EXPECT_EQ(FloatClipRect(), mapper.ClipRect());
 }
 
+TEST_F(ChunkToLayerMapperTest, SwitchToSiblingEffect) {
+  auto effect1 = CreateOpacityEffect(*LayerState().Effect(), 0.5f);
+  auto chunk1 = Chunk(PropertyTreeState(LayerState().Transform(),
+                                        LayerState().Clip(), effect1.get()));
+  auto effect2 = CreateOpacityEffect(*LayerState().Effect(), 0.5f);
+  auto chunk2 = Chunk(PropertyTreeState(LayerState().Transform(),
+                                        LayerState().Clip(), effect2.get()));
+
+  ChunkToLayerMapper mapper(chunk1.properties.GetPropertyTreeState(),
+                            gfx::Vector2dF(10, 20));
+  mapper.SwitchToChunk(chunk2);
+  EXPECT_FALSE(HasFilterThatMovesPixels(mapper));
+}
+
 }  // namespace blink