aura: WindowPortMus don't always create ClientSurfaceEmbedder

ClientSurfaceEmbedder was being created any time Window::SetEmbedFrameSinkId()
was being called. ClientSurfaceEmbedder is *only* needed if Embed() was also
called on this window. This patch makes it so ClientSurfaceEmbedder is only
created if Embed() is called.

BUG=none
TEST=none

Change-Id: I463458abb1248f1661358d4062389fa9a21f9446
Reviewed-on: https://chromium-review.googlesource.com/c/1369098
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615399}