Initialize stub platform window bounds to those of its mus::Window.

Fixes a regression from https://codereview.chromium.org/2009853002
IDK why it matters or if it relates to [still] broken window dragging.

TODO: Fix wallpaper picker dragging (not broken by same CL)...

BUG=557405
TEST=wallpaper picker shows for "Set wallpaper..." in mash.
R=markdittmer@chromium.org,sky@chromium.org,sadrul@chromium.org

Review-Url: https://codereview.chromium.org/2037013003
Cr-Commit-Position: refs/heads/master@{#397873}

ui/views/mus/window_tree_host_mus.cc

@@ -54,6 +54,8 @@ WindowTreeHostMus::WindowTreeHostMus(shell::Connector* connector,
   }
 
   SetPlatformWindow(base::WrapUnique(new ui::StubWindow(nullptr)));
+  // Initialize the stub platform window bounds to those of the mus::Window.
+  platform_window()->SetBounds(window->bounds());
 
   // The location of events is already transformed, and there is no way to
   // correctly determine the reverse transform. So, don't attempt to transform