Mark http/tests/devtools/debugger/fetch-breakpoints.js as slow.

This test seems slow - 20 iterations of the test took more than 80
seconds on Z840.  Therefore the [ Timeout ] expectation for this test
should probably be replaced with a [ Slow ] expectation.

Fixed: 678482
Change-Id: I8e400a813901d390673907bf9c7b64057fee5057
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137816
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757095}

diff --git a/third_party/blink/web_tests/SlowTests b/third_party/blink/web_tests/SlowTests
index 4572c51..efaa3cd 100644
--- a/third_party/blink/web_tests/SlowTests
+++ b/third_party/blink/web_tests/SlowTests

@@ -87,6 +87,7 @@
 crbug.com/902685 virtual/split-http-cache-not-site-per-process/http/tests/devtools/isolated-code-cache/* [ Slow ]
 # Misc DevTools tests that are slow
 crbug.com/243492 inspector-protocol/injected-script-discard.js [ Slow ]
+crbug.com/678482 http/tests/devtools/debugger/fetch-breakpoints.js [ Slow ]
 
 crbug.com/510337 cssom/cssvalue-comparison.html [ Slow ]