Add libgtk3.0 to CT setup-script.sh

Bug: chromium:990725
Change-Id: I397cf2b089dc401a16b1e363ac15112a8fb1f702
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/233465
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
diff --git a/go/gce/ct/setup-script.sh b/go/gce/ct/setup-script.sh
index 251d8d0..1c40701 100644
--- a/go/gce/ct/setup-script.sh
+++ b/go/gce/ct/setup-script.sh
@@ -10,7 +10,7 @@
 sudo apt-get update
 sudo apt-get -y install libosmesa-dev clang-3.6 poppler-utils netpbm \
     python-django libgif-dev lua5.2 libnss3 python-setuptools python-pip \
-    libglu1 xvfb
+    libglu1 libgtk3.0 xvfb
 sudo pip install -U crcmod mock psutil
 
 # Install openjdk-8. See skbug.com/6975 for context.