Add WindowTreeHostPlatform::GetHost for Android build

ui/aura/window_tree_host_platform.cc

Index: ui/aura/window_tree_host_platform.cc
diff --git a/ui/aura/window_tree_host_platform.cc b/ui/aura/window_tree_host_platform.cc
index 0ec5a8639e7472141f255133beacbbf86d8803d1..6108a3231364b1e25afaa8b3615c089c37c358cc 100644
--- a/ui/aura/window_tree_host_platform.cc
+++ b/ui/aura/window_tree_host_platform.cc
@@ -25,10 +25,26 @@
 #endif
 
 namespace aura {
+#if defined(OS_ANDROID)
+namespace {
+  WindowTreeHostPlatform* instance = nullptr;
+}
+
+// static
+WindowTreeHostPlatform* WindowTreeHostPlatform::GetHost() {
+  return instance;
+}
+#endif
 
 // static
 WindowTreeHost* WindowTreeHost::Create(const gfx::Rect& bounds) {
+#if defined(OS_ANDROID)
+  DCHECK(!instance);
+  instance = new WindowTreeHostPlatform(bounds);
+  return instance;
+#else
   return new WindowTreeHostPlatform(bounds);
+#endif
 }
 
 WindowTreeHostPlatform::WindowTreeHostPlatform(const gfx::Rect& bounds)