aura: moves WindowTrackerTemplate into WindowTracker

This code was split when we had two nearly identical window types. That is no
longer the case, so this template can be nuked.

BUG=none
TEST=none

Change-Id: I876e18d1335c74e2ab34e1ba34d423f60bf6babf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585160
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654483}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index d2169dbf..f23dded 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -157,6 +157,7 @@
     "window_port_for_shutdown.cc",
     "window_port_for_shutdown.h",
     "window_targeter.cc",
+    "window_tracker.cc",
     "window_tree_host.cc",
     "window_tree_host_platform.cc",
   ]