Stop preventing image layers from being marked as opaque.

Before https://chromium-review.googlesource.com/c/chromium/src/+/1038565
these layers would change their contents-opaque state when the
GraphicsLayer did, even though the image layer has its contents-opaque
set when setting the image.

Disallowing that to change created a memory regression, as it must be
that some images are not opaque, but the GraphicsLayer becomes opaque
later and the image layer should be marked as such too.

R=pdr@chromium.org

Bug: 838693, 841252
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I97c007b46d0bafcb9ecf19fad77327053e978aa2
Reviewed-on: https://chromium-review.googlesource.com/1069194Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560824}

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

@@ -1248,7 +1248,7 @@ void GraphicsLayer::SetContentsToImage(
   }
 
   SetContentsTo(image_layer_.get(),
-                /*prevent_contents_opaque_changes=*/true);
+                /*prevent_contents_opaque_changes=*/false);
 }
 
 cc::Layer* GraphicsLayer::CcLayer() const {