[iOS] Fix alignment of Keyboard Accessory View buttons.

The KAV buttons should be aligned with the ones from GBoard.

Bug: 
Change-Id: I8ba048719326ed93890fa989092cc460d0071e29
Reviewed-on: https://chromium-review.googlesource.com/579971
Commit-Queue: Jean-François Geyelin <jif@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488324}

ios/chrome/browser/ui/toolbar/new_keyboard_accessory_view.mm

@@ -59,7 +59,7 @@
   const CGFloat kButtonHeight = 34.0;
   const CGFloat kBetweenShortcutButtonSpacing = 5.0;
   const CGFloat kBetweenSearchButtonSpacing = 12.0;
-  const CGFloat kHorizontalMargin = 12.0;
+  const CGFloat kHorizontalMargin = 10.0;
 
   // Create and add stackview filled with the shortcut buttons.
   UIStackView* shortcutStackView = [[UIStackView alloc] init];