Fix the cell layout issue when creating new folder (iOS bookmarks)

Make the trailingAnchor of the stackView inside bookmarkTableCell always
equals to contentView's trailingAnchor.  This can prevent
crbug.com/778231 where the stackView's width doesn't grow as expected
when editing folder name on iOS 10.

Bug: 791976
Change-Id: I71fac7e335561fe7f54beb1062602976b0b2a280
Reviewed-on: https://chromium-review.googlesource.com/809988Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Marti Wong <martiw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522071}

ios/chrome/browser/ui/bookmarks/cells/bookmark_table_cell.mm

@@ -104,7 +104,7 @@ const CGFloat kBookmarkTableCellImagePadding = 16.0;
       [contentStack.leadingAnchor
           constraintEqualToAnchor:self.contentView.leadingAnchor],
       [contentStack.trailingAnchor
-          constraintLessThanOrEqualToAnchor:self.contentView.trailingAnchor]
+          constraintEqualToAnchor:self.contentView.trailingAnchor]
     ]];
 
     // Add separator view.