Update desk thumbnail radius on new spec.

Bug: 988517
Change-Id: I3643af0ada3f390440e9a65c28990bc43f92ae80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737328Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684130}

ash/wm/desks/desk_preview_view.cc

@@ -26,7 +26,7 @@ constexpr int kDeskPreviewHeight = 64;
 constexpr int kBorderSize = 2;
 
 // The rounded corner radii, also in dips.
-constexpr gfx::RoundedCornersF kCornerRadii(2);
+constexpr gfx::RoundedCornersF kCornerRadii(4);
 
 // Holds data about the original desk's layers to determine what we should do
 // when we attempt to mirror those layers.