Enable DispatchSafetyNetCheckOffThread by default

In preparation for an upcoming M64 launch. Note, this has implications
for Android Webview, which did not go through a fieldtrial. For
Webview, this only implements part of the experiment: the dispatching
of requests to the worker pool. Handling responses on a worker thread
can be implemented in a followup.

Bug: 786173
Change-Id: I089b64ab5333b4ff3bf60b5ad4c5c1659c65b896
Reviewed-on: https://chromium-review.googlesource.com/884001Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531758}

components/safe_browsing/features.cc

@@ -43,7 +43,7 @@ const base::Feature kTriggerThrottlerDailyQuotaFeature{
     base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kDispatchSafetyNetCheckOffThread{
-    "DispatchSafetyNetCheckOffThread", base::FEATURE_DISABLED_BY_DEFAULT};
+    "DispatchSafetyNetCheckOffThread", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kAppendRecentNavigationEvents{
     "AppendRecentNavigationEvents", base::FEATURE_DISABLED_BY_DEFAULT};