Increase time-out for rotate-interpolation test

The rotate-interpolation.html test occasionally flakes due to timeout.
This patch this test as slow test.
Link to the test flakiness:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests%20(with%20patch)&tests=external/wpt/css/css-transforms/animation/rotate-interpolation.html

Bug: 1067562
Change-Id: Ia75b5a1989deda052585b3c73df87bf2b48addbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2170412Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: Hao Sheng <haozhes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763453}

third_party/blink/web_tests/external/wpt/css/css-transforms/animation/rotate-interpolation.html

@@ -4,8 +4,9 @@
 <meta charset="utf-8">
 <title>rotate interpolation</title>
 <link rel="help" href="https://drafts.csswg.org/css-transforms-2/#propdef-rotate">
-
 <meta name="assert" content="rotate supports animation.">
+<meta name="timeout" content="long">
+
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script src="/css/support/interpolation-testcommon.js"></script>