[Sheriff] Mark 2 external/wpt tests as flaky on WebKit Linux MSAN

- external/wpt/secure-contexts/idlharness.any.html
- external/wpt/compression/compression-stream.tentative.any.serviceworker.html

Bug: 856601
Change-Id: If0e6187dc43163c3361bdd8a70ad2ee681af0e89
Tbr: csharp
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942888Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#720107}

third_party/blink/web_tests/MSANExpectations

@@ -354,3 +354,5 @@ crbug.com/856601 [ Linux ] virtual/controls-refresh/color/color-picker-manual-co
 crbug.com/856601 [ Linux ] virtual/omt-worker-fetch/external/wpt/fetch/cors-rfc1918/idlharness.tentative.any.html [ Pass Timeout ]
 
 # Note to sheriffs: Timeouts about "idlharness" or "interfaces" are usually crbug.com/856601.
+crbug.com/856601 [ Linux ] external/wpt/secure-contexts/idlharness.any.html [ Pass Timeout ]
+crbug.com/856601 [ Linux ] external/wpt/compression/compression-stream.tentative.any.serviceworker.html [ Pass Timeout ]