GestureNav: Make sure to always observe WindowAndroid

This CL fixes a potential bug of https://crrev.com/c/2371384.
OverscrollSceneLayer should always start observing WindowAndroid,
regardless of compositor attachment state.

Bug: 1120832
Change-Id: I9de81b89a4aacf08d74c53498e80e74ff358218a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2371725Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801584}

chrome/browser/android/compositor/scene_layer/overscroll_scene_layer.cc

@@ -18,8 +18,7 @@ OverscrollSceneLayer::OverscrollSceneLayer(JNIEnv* env,
     : SceneLayer(env, jobj),
       window_(ui::WindowAndroid::FromJavaWindowAndroid(jwindow)),
       glow_effect_(std::make_unique<ui::OverscrollGlow>(this)) {
-  if (window_->GetCompositor())
-    window_->AddObserver(this);
+  window_->AddObserver(this);
 }
 
 OverscrollSceneLayer::~OverscrollSceneLayer() {}