[ios] Improve toolbar in tab_grid

ios/clean/chrome/browser/ui/tab_grid/tab_grid_collection_view_layout.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/clean/chrome/browser/ui/tab_grid/tab_grid_collection_view_layout.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 namespace {
 const CGFloat kMinTabWidth = 200.0f;
 const CGFloat kMaxTabWidth = 250.0f;
 const CGFloat kInterTabSpacing = 20.0f;
-const UIEdgeInsets kSectionInset = {20.0f, 20.0f, 20.0f, 20.0f};
 }
 
 @implementation TabGridCollectionViewLayout
 
 #pragma mark - UICollectionViewLayout
 
 // This is called whenever the layout is invalidated, including during rotation.
 - (void)prepareLayout {
   [super prepareLayout];
   [self updateLayoutWithBounds:[[self collectionView] bounds].size];
 }
 
 #pragma mark - Private
 
 // This sets the appropriate itemSize given the bounds of the collection view.
 - (void)updateLayoutWithBounds:(CGSize)boundsSize {
   int columns = static_cast<int>(floor(boundsSize.width - kInterTabSpacing) /
                                  (kMinTabWidth + kInterTabSpacing));
   CGFloat tabWidth =
       (boundsSize.width - kInterTabSpacing * (columns + 1)) / columns;
   while (columns < 2 || tabWidth > kMaxTabWidth) {
     columns++;
     tabWidth = (boundsSize.width - kInterTabSpacing * (columns + 1)) / columns;
   }
   self.itemSize = CGSizeMake(tabWidth, tabWidth);
-  self.sectionInset = kSectionInset;
+  self.sectionInset = UIEdgeInsetsMake(10.0f, 20.0f, 20.0f, 20.0f);
   self.minimumLineSpacing = kInterTabSpacing;
   self.minimumInteritemSpacing = kInterTabSpacing;
 }
 
 @end