Views/Harmony: Use LayoutDelegate::GetMetric for views::kPanelVertMargin.

chrome/browser/ui/views/webshare/webshare_target_picker_view.cc

Index: chrome/browser/ui/views/webshare/webshare_target_picker_view.cc
diff --git a/chrome/browser/ui/views/webshare/webshare_target_picker_view.cc b/chrome/browser/ui/views/webshare/webshare_target_picker_view.cc
index 3b246991291282d0561078563a59e6ae0990c8c0..cf1b45835343214f7df1c0563f7177f06ada1ef2 100644
--- a/chrome/browser/ui/views/webshare/webshare_target_picker_view.cc
+++ b/chrome/browser/ui/views/webshare/webshare_target_picker_view.cc
@@ -80,11 +80,11 @@ WebShareTargetPickerView::WebShareTargetPickerView(
     : targets_(targets),
       table_model_(base::MakeUnique<TargetPickerTableModel>(&targets_)),
       close_callback_(close_callback) {
-  views::BoxLayout* layout = new views::BoxLayout(
-      views::BoxLayout::kVertical,
-      LayoutDelegate::Get()->GetMetric(
-          LayoutDelegate::Metric::PANEL_CONTENT_MARGIN),
-      views::kPanelVertMargin, views::kRelatedControlVerticalSpacing);
+  const int panel_margin = LayoutDelegate::Get()->GetMetric(
+      LayoutDelegate::Metric::PANEL_CONTENT_MARGIN);
+  views::BoxLayout* layout =
+      new views::BoxLayout(views::BoxLayout::kVertical, panel_margin,
+                           panel_margin, views::kRelatedControlVerticalSpacing);

[tapted, 2017/02/27 03:24:51]

Could we have a 

BoxLayout* LayoutDelegate::CreatePanelBoxLayout(..)

(i.e. similar to GridLayout::CreatePanel)

(we'll probably also want LayoutDelegate::CreatePanelGridLayout..)

[Peter Kasting, 2017/02/28 02:21:39]

https://bugs.chromium.org/p/chromium/issues/detail?id=687340 is about trying to
figure out the grid layout stuff for panels, let's not create a third place to
unify into there :).

My inclination is to do this as a BoxLayout member, if it's desirable.  It's
sort of "BoxLayout-specific" more than it is "LayoutDelegate-specific".

As to whether it's desirable, I don't know.  How often will it be used?  It
seems like a lot of places in this CL are each doing a one-off thing.

[Patti Lor, 2017/03/03 08:24:39]

Did a quick grep for places creating new BoxLayouts - I think a lot of places
are creating them with 0s for all the spacing arguments, so not sure if that
would be used a lot in places that want default panel margins. Because of that
I've left it as is, so if either of you feels strongly about it let me know -
I'm happy to defer to you both on this.

   SetLayoutManager(layout);
 
   views::Label* overview_label = new views::Label(