Mark persistence-external-change-breakpoints.js as Slow.

Bug: 1040450
Change-Id: I80fa1ff683706760207e37c00e22b66375cce23e
No-Try: true
Tbr: vasilii@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995104Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730099}

third_party/blink/web_tests/SlowTests

@@ -674,3 +674,4 @@ crbug.com/1027439 http/tests/devtools/storage-panel-dom-storage-update.js [ Slow
 crbug.com/1020379 http/tests/devtools/a11y-axe-core/* [ Slow ]
 
 crbug.com/1040054 http/tests/devtools/cache-storage/cache-data.js [ Slow ]
+crbug.com/1040450 http/tests/devtools/persistence/persistence-external-change-breakpoints.js [ Slow ]