Fix issue that gesture scroll is not responsive

The type of scroll offset should be float instead of int to prevent the
decimal part from being cut.

Bug: 1050812
Change-Id: Id6ad2ba279c0bd6b48308a3d27de271c8a8ff70a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047991Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740118}

ash/shelf/scrollable_shelf_view.cc

@@ -1557,7 +1557,7 @@ bool ScrollableShelfView::ProcessGestureEvent(const ui::GestureEvent& event) {
   DCHECK(presentation_time_recorder_);
   presentation_time_recorder_->RequestNext();
 
-  const int scroll_x = -event.details().scroll_x();
+  const float scroll_x = -event.details().scroll_x();
   if (GetShelf()->IsHorizontalAlignment()) {
     ScrollByXOffset(ShouldAdaptToRTL() ? -scroll_x : scroll_x,
                     /*animate=*/false);