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}

• blimp/common/proto/blimp_message.proto[diff]
• blimp/common/proto/input.proto[diff]
• blimp/common/proto/tab_control.proto[diff]
• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/layers/empty_content_layer_client.cc[Added - diff]
• cc/layers/empty_content_layer_client.h[Added - diff]
• cc/layers/layer.cc[diff]
• cc/layers/layer_proto_converter.cc[diff]
• cc/layers/layer_proto_converter_unittest.cc[diff]
• cc/layers/picture_layer.cc[diff]
• cc/layers/picture_layer.h[diff]
• cc/layers/picture_layer_unittest.cc[diff]
• cc/playback/display_list_recording_source.cc[diff]
• cc/playback/display_list_recording_source_unittest.cc[diff]
• cc/proto/layer.proto[diff]
• cc/test/fake_display_list_recording_source.cc[diff]