WebUI Tab Strip: Push tab group's drag image down

On line 89, the drag image has a padding and is translated up, which
causes the drag image to peak up a bit while the group is being
dragged. This is more noticeable when there's a scrollbar.

Before and after: https://imgur.com/a/p6UH8e9

Change-Id: If75024d5b3dec3ceffa7a82bdaf6ab52172fa0c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292860Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787830}

chrome/browser/resources/tab_strip/tab_group.html

@@ -164,7 +164,7 @@
 :host([dragging]) #dragImage {
   /* Position the actual drag image out of view so it is not visible. */
   position: absolute;
-  top: 100vh;
+  top: calc(100vh + var(--drag-image-padding));
 }
 </style>