Fix ripple on cloud import button

Paper ripples need an ancestor with style `position: relative` in order
for the ripple to not escape.

Bug: 1014819
Change-Id: I1f07fb769a47c573bafebf96172d5e2cfe309b96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863058
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706320}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -461,6 +461,10 @@ body:not(.check-select) #selection-menu-button {
   display: none;
 }
 
+.cloud-import-combo-button {
+  position: relative;
+}
+
 .cloud-import-combo-button > .buttons {
   display: flex;
 }