Revert "overview: Close button is not layouted properly."

This reverts commit 851088cf.

Reason for revert: This is wrong fix.

Original change's description:
> overview: Close button is not layouted properly.
> 
> Gave the parent a rect that has x-bounds, but still using the parent's
> parent width, resulting in a overflow.
> 
> Test: manual
> Bug: 1020539
> Change-Id: I983cf0813c6b99a74960761c8c8fd241b2995e0e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914942
> Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#715105}

TBR=afakhry@chromium.org,sammiequon@chromium.org

Change-Id: Iea026b292493d8c9ca0931d6a40606e053bda2b8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1020539
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914796Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715151}

ash/wm/overview/overview_item_view.cc

@@ -269,8 +269,7 @@ gfx::Rect OverviewItemView::GetHeaderBounds() const {
   // ---------------------------+---------+
   //                                   c
   //                                   |
-  return gfx::Rect(margin, margin,
-                   GetLocalBounds().width() - margin - right_padding,
+  return gfx::Rect(margin, margin, GetLocalBounds().width() - right_padding,
                    kHeaderHeightDp);
 }