[ios] Fixes the implementation of [BookmarkTestCase scrollToTop].

This method was incorrectly invoking an EarlGrey method that scrolled to
the bottom of the view, not the top.

BUG=853330

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic5bfe4be02594828c4c12a45e3dab78a51641cac
Reviewed-on: https://chromium-review.googlesource.com/1104382Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568020}

ios/chrome/browser/ui/bookmarks/bookmarks_egtest.mm

@@ -3748,8 +3748,8 @@ id<GREYMatcher> TappableBookmarkNodeWithLabel(NSString* label) {
   // certain configurations.
   [[EarlGrey
       selectElementWithMatcher:grey_accessibilityID(@"bookmarksTableView")]
-      performAction:grey_scrollToContentEdgeWithStartPoint(
-                        kGREYContentEdgeBottom, 0.5, 0.5)];
+      performAction:grey_scrollToContentEdgeWithStartPoint(kGREYContentEdgeTop,
+                                                           0.5, 0.5)];
 }
 
 // Scroll the bookmarks to bottom.