Snap dropped tabs to split view, when appropriate

This also adds unit test coverage for all of TabDragDropDelegate.

Bug: 1069869
Change-Id: I719586ffcc775b3a7ea5eab1fc8d38d5baaa665d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2176675
Reviewed-by: Mitsuru Oshima <[email protected]>
Reviewed-by: Xiaoqian Dai <[email protected]>
Commit-Queue: Collin Baker <[email protected]>
Cr-Commit-Position: refs/heads/master@{#766563}
diff --git a/ash/shell_delegate.cc b/ash/shell_delegate.cc
index 684e8fc8..6dccd69 100644
--- a/ash/shell_delegate.cc
+++ b/ash/shell_delegate.cc
@@ -6,10 +6,10 @@
 
 namespace ash {
 
-bool ShellDelegate::CreateBrowserForTabDrop(
-    gfx::NativeWindow source_window,
+aura::Window* ShellDelegate::CreateBrowserForTabDrop(
+    aura::Window* source_window,
     const ui::OSExchangeData& drop_data) {
-  return false;
+  return nullptr;
 }
 
 media_session::mojom::MediaSessionService*