Mark wpt/.../execution-timing/078.html as slow

Bug: 842995
Change-Id: Ia322131520ba004da8b6ab1e8c4c749971625006
Tbr: foolip@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1060024
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558778}

diff --git a/third_party/WebKit/LayoutTests/SlowTests b/third_party/WebKit/LayoutTests/SlowTests
index 3af4c32..db79330 100644
--- a/third_party/WebKit/LayoutTests/SlowTests
+++ b/third_party/WebKit/LayoutTests/SlowTests

@@ -515,3 +515,5 @@
 # This test does a lot of IPC because it tests limits on IPC allocations,
 # and is therefore slow.
 crbug.com/826957 fast/peerconnection/RTCPeerConnection-manyCandidates.html [ Slow ]
+
+crbug.com/842995 external/wpt/html/semantics/scripting-1/the-script-element/execution-timing/078.html [ Slow ]