Remove needless function call when starting tab dragging

PrepareForAnimation does nothing when tab_drag_controller is
set (tab dragging is started), so we can safely remove its
needless call from TabStrip::StartedDraggingTabs method, which
is called from TabDragController::Attach.

R=sky@chromium.org

Change-Id: I0b084512d2ee2804cf602d2a4e71bea2fd7c80ff
Bug: 896640
Reviewed-on: https://chromium-review.googlesource.com/c/1489214Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#635952}

chrome/browser/ui/views/tabs/tab_strip.cc

@@ -2032,8 +2032,6 @@ void TabStrip::StartedDraggingTabs(const Tabs& tabs) {
   if (MayHideNewTabButtonWhileDragging() && !drag_controller_)
     new_tab_button_->SetVisible(false);
 
-  PrepareForAnimation();
-
   // Reset dragging state of existing tabs.
   for (int i = 0; i < tab_count(); ++i)
     tab_at(i)->set_dragging(false);