Do not mark directly composited image layers as opaque

Due to https://crbug.com/870857, we cannot mark directly composited
image layers as opaque. https://crrev.com/556288 started setting
prevent_contents_opaque_changes_ for directly-composited image layers
which exposed this bug.

Bug: 868779
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9fb0fe4361bd8fd39529bbf698f7549edbc51faf
Reviewed-on: https://chromium-review.googlesource.com/1162750
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580725}

third_party/blink/renderer/platform/graphics/graphics_layer.cc

@@ -876,7 +876,8 @@ void GraphicsLayer::SetContentsToImage(
     }
     image_layer_->SetImage(std::move(paint_image), matrix,
                            image_orientation.UsesWidthAsHeight());
-    image_layer_->SetContentsOpaque(image->CurrentFrameKnownToBeOpaque());
+    // Image layers can not be marked as opaque due to crbug.com/870857.
+    image_layer_->SetContentsOpaque(false);
     UpdateContentsRect();
   } else if (image_layer_) {
     UnregisterContentsLayer(image_layer_.get());

third_party/blink/renderer/platform/graphics/image_layer_chromium_test.cc

@@ -131,7 +131,9 @@ TEST(ImageLayerChromiumTest, opaqueImages) {
 
   graphics_layer->SetContentsToImage(opaque_image.get(),
                                      Image::kUnspecifiedDecode);
-  ASSERT_TRUE(graphics_layer->ContentsLayer()->contents_opaque());
+  // This would normally have contents_opaque set but due to crbug.com/870857,
+  // we cannot set image layers as having contents_opaque.
+  ASSERT_FALSE(graphics_layer->ContentsLayer()->contents_opaque());
 
   graphics_layer->SetContentsToImage(non_opaque_image.get(),
                                      Image::kUnspecifiedDecode);