Ash Tray: Fix bubble overflowing in tablet overview mode.

BUG=1042946

Change-Id: I665dbfa33d041c870a18433c7f69225cc4e00c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006192
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732669}

ash/system/tray/tray_utils.cc

@@ -61,7 +61,8 @@ gfx::Insets GetTrayBubbleInsets() {
   int height_compensation = kTrayBubbleInsetHotseatCompensation;
   switch (shelf->GetBackgroundType()) {
     case ShelfBackgroundType::kInApp:
-      // In-app mode does not require a height compensation.
+    case ShelfBackgroundType::kOverview:
+      // Certain modes do not require a height compensation.
       height_compensation = 0;
       break;
     case ShelfBackgroundType::kLogin: