Mark cache-data.js as Slow for causing flaky timeouts.

Bug: 1040054
Change-Id: Iea5bfbcec4710b766b4b5117c188aa870869266e
No-Try: true
Tbr: vasilii@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995081Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730092}

third_party/blink/web_tests/SlowTests

@@ -672,3 +672,5 @@ crbug.com/1027439 http/tests/devtools/storage-panel-dom-storage-update.js [ Slow
 
 # Mark all axe audit tests slow.
 crbug.com/1020379 http/tests/devtools/a11y-axe-core/* [ Slow ]
+
+crbug.com/1040054 http/tests/devtools/cache-storage/cache-data.js [ Slow ]