cros: Fix context menu positioning for left shelf

Fix a simple error that I made in the context menu positioning logic.

Bug: 838389
Change-Id: I2ee54d111a47bfa101af7db2fa8593ca26275a73
Reviewed-on: https://chromium-review.googlesource.com/1050509
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556910}

ui/views/controls/menu/menu_controller.cc

@@ -2246,7 +2246,7 @@ gfx::Rect MenuController::CalculateBubbleMenuBounds(MenuItemView* item,
       y = owner_bounds.origin().y() - border_and_shadow_insets.top();
       // Align the left of the menu with the right of the anchor.
       if (x < 0) {
-        x = owner_bounds.right() + border_and_shadow_insets.left() +
+        x = owner_bounds.right() - border_and_shadow_insets.left() +
             menu_config.touchable_anchor_offset;
       }
       // Align the bottom of the menu to the bottom of the anchor.