Limit CompositorFrameMetadata::selection to Clank

Platforms other than Android, use selection sent in RenderFrameMetadata
rather than the one in CompositorFrameMetadata. There is no need to
serialize/deserialize and send the unused data on each frame.

BUG=859969

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ibfde9270deae40ebc57e15251b70597fd3e6a0f7
Reviewed-on: https://chromium-review.googlesource.com/1172142
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582626}

• cc/trees/layer_tree_host_impl.cc[diff]
• cc/trees/layer_tree_host_impl_unittest.cc[diff]
• components/viz/common/quads/compositor_frame_metadata.h[diff]
• content/common/content_param_traits_macros.h[diff]
• services/viz/public/cpp/compositing/compositor_frame_metadata_struct_traits.cc[diff]
• services/viz/public/cpp/compositing/compositor_frame_metadata_struct_traits.h[diff]
• services/viz/public/cpp/compositing/struct_traits_unittest.cc[diff]
• services/viz/public/interfaces/compositing/compositor_frame_metadata.mojom[diff]