Serialize PictureLayer properties.

Serialize the properties specific to the PictureLayer subclass.  This
required a few slight changes:
- Don't query LayerImpl::bounds() in PictureLayer::PushPropertiesTo().
  Instead rely on either Layer::bounds() or
  Layer::paint_properties().bounds depending on the
  source_frame_number.  Since LayerImpl's bounds are set by
  Layer::PushPropertiesTo() right before this check, this should
  result in the same behavior.
- Create an EmptyContentLayerClient to be used by all PictureLayer
  instances created by
  LayerProtoConverter::FindOrAllocateAndConstruct().

BUG=538710
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1527863002

Cr-Commit-Position: refs/heads/master@{#367300}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 4f1b235..88fff04 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -123,6 +123,8 @@
     "layers/delegated_renderer_layer_impl.h",
     "layers/draw_properties.cc",
     "layers/draw_properties.h",
+    "layers/empty_content_layer_client.cc",
+    "layers/empty_content_layer_client.h",
     "layers/heads_up_display_layer.cc",
     "layers/heads_up_display_layer.h",
     "layers/heads_up_display_layer_impl.cc",