Add CSS Transitions performance tests

tools/perf/page_sets/tough_animation_cases/resources/tablet.css

Index: tools/perf/page_sets/tough_animation_cases/resources/tablet.css
diff --git a/tools/perf/page_sets/tough_animation_cases/resources/tablet.css b/tools/perf/page_sets/tough_animation_cases/resources/tablet.css
new file mode 100644
index 0000000000000000000000000000000000000000..95c53fbe6c05e8e7eebe7a41cf685588deaeb870
--- /dev/null
+++ b/tools/perf/page_sets/tough_animation_cases/resources/tablet.css
@@ -0,0 +1,24 @@
+/* Copyright 2014 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+body {
+  margin: 0;
+  overflow: hidden;
+}
+
+container {
+  display: flex;
+  flex-flow: row wrap;
+  width: 550px;
+  height: 800px;
+  background-color: lightgrey;
+}
+
+target {
+  display: inline-block;
+  width: 11px;
+  height: 11px;
+  background: green;
+}