MSan: temporarily blackist a function in sandbox_linux_unittests.

This code triggers an MSan issue where we lack an interceptor for
sched_getparam().

BUG=418986
TBR=timurrrr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/614063002

Cr-Commit-Position: refs/heads/master@{#297419}

tools/msan/blacklist.txt

@@ -20,3 +20,6 @@ fun:*UTF8GenericReplaceInternal*
 # False positives due to use of linux_syscall_support. http://crbug.com/394028
 src:*/breakpad/src/*
 src:*/components/crash/app/breakpad_linux.cc
+
+# False positives due to an MSan bug. http://crbug.com/418986
+fun:*SchedGetParamThread*