Mark wpt/.../euckr-decode-ksc_5601.html as flaky.

external/wpt/encoding/legacy-mb-korean/euc-kr/euckr-decode-ksc_5601.html
started failing recently on some Mac bots. Marking as flaky per WPT
import instructions.

Bug: 865004
Change-Id: I7f0fd0e3af4fbd5fce041456c906facf279a8c0b
Tbr: jsbell@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1141926Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576061}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4639,4 +4639,4 @@ crbug.com/864891 [ Mac ] virtual/scroll_customization/fast/scroll-behavior/scrol
 
 # Sheriff 2018-07-18
 crbug.com/863067 [ Win10 ] fast/dom/Window/window-focus-self.html [ Failure Pass ]
-crbug.com/864994 [ Mac ] external/wpt/encoding/legacy-mb-korean/euc-kr/euckr-decode-ksc_5601.html [ Failure Pass ]
+crbug.com/864994 [ Mac ] external/wpt/encoding/legacy-mb-korean/euc-kr/euckr-decode-ksc_5601.html [ Timeout Failure Pass ]