Enable holding space previews in shelf by default.

When holding space is enabled and the holding space has content, file
thumbnails will show in the shelf. The user can enable/disable this
behavior via context menu.

Bug: 1155387
Change-Id: Ie7e6a56d677473ca19b70b98bb383db082ace2b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2605397Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#840238}

ash/public/cpp/ash_features.cc

@@ -147,7 +147,7 @@ const base::Feature kTemporaryHoldingSpace{"TemporaryHoldingSpace",
                                            base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kTemporaryHoldingSpacePreviews{
-    "TemporaryHoldingSpacePreviews", base::FEATURE_DISABLED_BY_DEFAULT};
+    "TemporaryHoldingSpacePreviews", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kDragUnpinnedAppToPin{"DragUnpinnedAppToPin",
                                           base::FEATURE_DISABLED_BY_DEFAULT};