Fix a checkerboad related crash.

For a mask layer, the color's alpha channel has to be 255, otherwise
a DCHECK() in cc::RenderSurfaceImpl::TileMaskLayer() will be triggered.

Bug: 897731
Change-Id: I9281ebdd1eccde58642297618051b2a1fb0a81e7
Reviewed-on: https://chromium-review.googlesource.com/c/1349454Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610681}

cc/layers/picture_layer_impl.cc

@@ -526,7 +526,8 @@ void PictureLayerImpl::AppendQuads(viz::RenderPass* render_pass,
     if (!has_draw_quad) {
       // Checkerboard.
       SkColor color = SafeOpaqueBackgroundColor();
-      if (ShowDebugBorders(DebugBorderType::LAYER)) {
+      if (mask_type_ == Layer::LayerMaskType::NOT_MASK &&
+          ShowDebugBorders(DebugBorderType::LAYER)) {
         // Fill the whole tile with the missing tile color.
         color = DebugColors::OOMTileBorderColor();
       }