DevTools: eagerly marking certain DevTools tests as slow

After landing a batch of migrated tests, one of the tests got marked as flaky
and then got reverted:
* crbug.com/751906
* https://chromium-review.googlesource.com/c/599828

Bug: 667560
Change-Id: Ie5a819ca59b428a7a009a6ff21b3b66768ba0cd4
Reviewed-on: https://chromium-review.googlesource.com/600829Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Will Chen <chenwilliam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491831}

third_party/WebKit/LayoutTests/SlowTests

@@ -104,6 +104,17 @@ crbug.com/451577 [ Mac ] inspector/network/network-domain-filter.html [ Slow ]
 
 crbug.com/237245 [ Debug Win7 ] editing/selection/programmatic-selection-on-mac-is-directionless.html [ Slow ]
 
+#############################################################
+# Marking migrated DevTools tests slow based on existing bugs
+# See crbug.com/667560 for details
+
+# Most DevTools tests are slow in Debug.
+webkit.org/b/90488 [ Debug ] http/tests/devtools/ [ Slow ]
+webkit.org/b/90488 [ Debug ] virtual/mojo-loading/http/tests/devtools/ [ Slow ]
+crbug.com/451577 http/tests/devtools/console [ Slow ]
+
+#############################################################
+
 # Most inspector tests are slow in Debug.
 webkit.org/b/90488 [ Debug ] inspector/ [ Slow ]
 webkit.org/b/90488 [ Debug ] http/tests/inspector/ [ Slow ]