Mark structured-clone test as Slow, not Timeout

Per discussion in https://codereview.chromium.org/2465483002 a more
appropriate expectation for this test is [Slow], not [Timeout].

BUG=660492
[email protected]
NOTRY=true

Review-Url: https://codereview.chromium.org/2474273002
Cr-Commit-Position: refs/heads/master@{#429730}
diff --git a/third_party/WebKit/LayoutTests/SlowTests b/third_party/WebKit/LayoutTests/SlowTests
index b33512f..f7191b407 100644
--- a/third_party/WebKit/LayoutTests/SlowTests
+++ b/third_party/WebKit/LayoutTests/SlowTests
@@ -350,3 +350,5 @@
 # IndexedDB, which can be slow.
 crbug.com/660468 [ Linux ] storage/indexeddb/observer-frame.html [ Slow ]
 crbug.com/660468 [ Linux ] storage/indexeddb/observer-workers.html [ Slow ]
+
+crbug.com/660492 [ Linux ] storage/indexeddb/structured-clone.html [ Slow ]