Disable urgent tab discarding on Linux

Bug: 1155316
Change-Id: I70e84f9445edae16c4fcab6d03b33a8f958ba842
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572888Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833592}

chrome/browser/performance_manager/policies/policy_features.cc

@@ -125,7 +125,7 @@ const base::Feature kPageFreezingFromPerformanceManager{
 
 const base::Feature kUrgentDiscardingFromPerformanceManager{
   "UrgentDiscardingFromPerformanceManager",
-#if BUILDFLAG(IS_CHROMEOS_ASH)
+#if BUILDFLAG(IS_CHROMEOS_ASH) || defined(OS_LINUX)
       base::FEATURE_DISABLED_BY_DEFAULT
 #else
       base::FEATURE_ENABLED_BY_DEFAULT