App list: The line under folder headings has a fixed margin.

Previously, it had a fixed width, causing it to be very short on the
wider app list window style. Now it extends (almost) all the way to the
edge regardless of the app list's width.

BUG=415461

Review URL: https://codereview.chromium.org/581853003

Cr-Commit-Position: refs/heads/master@{#295697}

ui/app_list/views/folder_header_view.cc

@@ -27,7 +27,7 @@ const int kPreferredWidth = 360;
 const int kPreferredHeight = 48;
 const int kIconDimension = 24;
 const int kPadding = 14;
-const int kBottomSeparatorWidth = 380;
+const int kBottomSeparatorPadding = 9;
 const int kBottomSeparatorHeight = 1;
 const int kMaxFolderNameWidth = 300;
 
@@ -192,9 +192,8 @@ void FolderHeaderView::OnPaint(gfx::Canvas* canvas) {
     return;
 
   // Draw bottom separator line.
-  rect.set_x((rect.width() - kBottomSeparatorWidth) / 2 + rect.x());
-  rect.set_y(rect.y() + rect.height() - kBottomSeparatorHeight);
-  rect.set_width(kBottomSeparatorWidth);
+  rect.Inset(kBottomSeparatorPadding, 0);
+  rect.set_y(rect.bottom() - kBottomSeparatorHeight);
   rect.set_height(kBottomSeparatorHeight);
   canvas->FillRect(rect, kTopSeparatorColor);
 }