[ios] Fixes a crash when displaying the downloads UI after a side swipe.

During a side swipe to change tabs, we remove the content area from the
view hierarchy.  This was causing issues with the downloads UI, as we
were attempting to set up constraints before the view hierarchy was
reassembled.  This CL swaps the order of calls to
sideSwipeViewDismissAnimationDidEnd and setCurrentTab, to ensure that
the view hierarchy is fully reassembled before any of the tab change
machinery is invoked.

BUG=849827

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iba2642f949a10d1a31ed32ae3763fed244022fdf
Reviewed-on: https://chromium-review.googlesource.com/1138351
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575664}
1 file changed