Add libgtk-3-0:i386 to backwards-compatible list

The package was removed from nacl_list in r607986.

BUG=None
TBR=dpranke

Change-Id: I052e3a0fdc5f58834bfec9b38502b5aa60e2edf0
Reviewed-on: https://chromium-review.googlesource.com/c/1354353Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611947}

build/install-build-deps.sh

@@ -289,6 +289,7 @@ backwards_compatible_list="\
   libgl1-mesa-dev
   libgl1-mesa-glx:i386
   libgles2-mesa-dev
+  libgtk-3-0:i386
   libgtk2.0-0
   libgtk2.0-0:i386
   libgtk2.0-dev