Add VectorIconButton functionality to ImageButton.

chrome/browser/ui/views/harmony/layout_delegate.h

Index: chrome/browser/ui/views/harmony/layout_delegate.h
diff --git a/chrome/browser/ui/views/harmony/layout_delegate.h b/chrome/browser/ui/views/harmony/layout_delegate.h
index e37879f86ed5e5433edaf45c5a8b4f31ef58058a..70de554f68f449e9b8b17966860a6ea87f6ae5b8 100644
--- a/chrome/browser/ui/views/harmony/layout_delegate.h
+++ b/chrome/browser/ui/views/harmony/layout_delegate.h
@@ -12,6 +12,11 @@ class LayoutDelegate {
   enum class Metric {
     // Padding on the left and right side of a button's label.
     BUTTON_HORIZONTAL_PADDING,
+    // Extra margin added on all sides of a button's content.  This is only
+    // used for certain button types.
+    // TODO(pkasting): This should probably be touch-specific and needs to be
+    // considered more holistically.
+    BUTTON_MARGIN,
     // The maximum width a button can have and still influence the sizes of
     // other linked buttons.  This allows short buttons to have linked widths
     // without long buttons making things overly wide.