Add service-worker-v8-cache.js to SlowTests

To know whether the test has a bug or the test is just slow.
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=service-worker-v8-cache.js

Bug: 789111
Change-Id: I7aa771ca9712882c261e46d6af48241dac465a0c
Reviewed-on: https://chromium-review.googlesource.com/835967Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525280}

third_party/WebKit/LayoutTests/SlowTests

@@ -123,6 +123,7 @@ crbug.com/451577 [ Mac ] http/tests/devtools/extensions/extensions-resources.js
 crbug.com/451577 [ Win10 ] http/tests/devtools/extensions/extensions-sidebar.js [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/layers/layer-canvas-log.js [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/network/network-domain-filter.js [ Slow ]
+crbug.com/789111 virtual/pwa-full-code-cache/http/tests/devtools/service-workers/service-worker-v8-cache.js [ Slow ]
 
 # Random slow tests
 crbug.com/763197 [ Linux Mac ] virtual/gpu-rasterization/images/color-profile-border-radius.html [ Slow ]