Converts MruWindowTracker to work with common types

ash/common/wm_window.h

Index: ash/common/wm_window.h
diff --git a/ash/common/wm_window.h b/ash/common/wm_window.h
index 8049a09804f05d42a39ce2fc51b5c6093207f960..14cec7f071a882ee2c96880060fc7587af2a2c1f 100644
--- a/ash/common/wm_window.h
+++ b/ash/common/wm_window.h
@@ -54,6 +54,8 @@ class ASH_EXPORT WmWindow {
     USE_SCREEN_COORDINATES,
   };
 
+  using Windows = std::vector<WmWindow*>;

[James Cook, 2016/06/07 01:21:05]

optional: I know this is style-guide compliant, and I know we do it in lots of
places, but I'm not a fan. I think it's important to show, at the call site
manipulating the windows, the type of the container (set? vector? other?), since
it affects both performance and what STL API functions you can use (insert?
push_back?).

[sky, 2016/06/07 02:46:21]

I left it for the following reasons:
. as you say, this style is used in plenty of places.
. when performance matters you have to know the type and will look it up.

[James Cook, 2016/06/07 14:05:38]

still optional: How about WindowList as a name, then?

[sky, 2016/06/07 15:34:21]

I went with Windows because of:
https://cs.chromium.org/chromium/src/ui/aura/window.h?rcl=0&l=85 .

+
   WmWindow* GetRootWindow() {
     return const_cast<WmWindow*>(
         const_cast<const WmWindow*>(this)->GetRootWindow());
@@ -132,7 +134,7 @@ class ASH_EXPORT WmWindow {
         const_cast<const WmWindow*>(this)->GetTransientParent());
   }
   virtual const WmWindow* GetTransientParent() const = 0;
-  virtual std::vector<WmWindow*> GetTransientChildren() = 0;
+  virtual Windows GetTransientChildren() = 0;
 
   virtual void SetLayoutManager(
       std::unique_ptr<WmLayoutManager> layout_manager) = 0;
@@ -227,7 +229,7 @@ class ASH_EXPORT WmWindow {
   virtual void StackChildAbove(WmWindow* child, WmWindow* target) = 0;
   virtual void StackChildBelow(WmWindow* child, WmWindow* target) = 0;
 
-  virtual std::vector<WmWindow*> GetChildren() = 0;
+  virtual Windows GetChildren() = 0;
 
   // Shows/hides the resize shadow. |component| is the component to show the
   // shadow for (one of the constants in ui/base/hit_test.h).