views: only compile DesktopWindowTreeHostPlatform on linux

That's the only place it's currently used, and I'm tired of waiting on
windows try bots. If we decide to use on windows we can move back to
other section.

BUG=none
TEST=none

Change-Id: Id1ff274daa25ea51ceef796948a1db9f0a5f60b9
Reviewed-on: https://chromium-review.googlesource.com/1004116Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549578}

ui/views/BUILD.gn

@@ -594,7 +594,6 @@ jumbo_component("views") {
       "widget/desktop_aura/desktop_screen.h",
       "widget/desktop_aura/desktop_screen_position_client.h",
       "widget/desktop_aura/desktop_window_tree_host.h",
-      "widget/desktop_aura/desktop_window_tree_host_platform.h",
       "widget/focus_manager_event_handler.h",
       "widget/native_widget_aura.h",
       "widget/tooltip_manager_aura.h",
@@ -629,7 +628,6 @@ jumbo_component("views") {
       "widget/desktop_aura/desktop_native_cursor_manager.cc",
       "widget/desktop_aura/desktop_native_widget_aura.cc",
       "widget/desktop_aura/desktop_screen_position_client.cc",
-      "widget/desktop_aura/desktop_window_tree_host_platform.cc",
       "widget/focus_manager_event_handler.cc",
       "widget/native_widget_aura.cc",
       "widget/tooltip_manager_aura.cc",
@@ -698,6 +696,11 @@ jumbo_component("views") {
     }
   }
 
+  if (is_linux) {
+    sources += [ "widget/desktop_aura/desktop_window_tree_host_platform.cc" ]
+    public += [ "widget/desktop_aura/desktop_window_tree_host_platform.h" ]
+  }
+
   if (is_mac) {
     sources -= [ "controls/views_text_services_context_menu.cc" ]
     deps += [