Make the extra padding around VectorIconButtons configurable.

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 9841b67c9e3f24dfff94c554e74ac90936bda2eb..51498f38a39b1d902e6a56f5371c9eeaadfbe225 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,
     // Default minimum width of a button.
     BUTTON_MINIMUM_WIDTH,
     // Margin between the edge of a dialog and the left, right, or bottom of a