[PM] On ChromeOS disable TrimWorkingSet on freeze by default.

WorkingSetTrim on ChromeOS involved new kernel changes, to avoid
potential issues related to experimentation the Catan team may be
doing we will disable this feature by default on ChromeOS.

TrimWorkingSet on ChromeOS involved a fairly large kernel change,
the concern here is that if we made a mistake that causes kernel
panics we may not be watching the right metric or aware that it's
being exercised.

BUG=chromium:973963

Change-Id: I8b0702b61bb92519c077762ebd29297e615e6ba7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758603Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689085}

chrome/browser/performance_manager/graph/policies/policy_features.cc

@@ -19,7 +19,7 @@ const base::Feature kTrimOnMemoryPressure{"TrimOnMemoryPressure",
                                           base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kTrimOnFreeze{"TrimOnFreeze",
-                                  base::FEATURE_ENABLED_BY_DEFAULT};
+                                  base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::FeatureParam<int> kGraphWalkBackoffTimeSec = {
     &kTrimOnMemoryPressure, "GraphWalkBackoffTimeSec", 120};