Add a feature knob for task scheduler priority of system DNS

Bug: 811272
Change-Id: I9d7a98ee36cf5d19dfb13a770eb197a09a2309ff
Reviewed-on: https://chromium-review.googlesource.com/935422Reviewed-by: Miriam Gershenson <mgersh@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539152}

net/dns/host_resolver_impl.cc

@@ -36,6 +36,7 @@
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/field_trial_params.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
@@ -291,6 +292,15 @@ bool ConfigureAsyncDnsNoFallbackFieldTrial() {
   return kDefault;
 }
 
+const base::FeatureParam<base::TaskPriority>::Option prio_modes[] = {
+    {base::TaskPriority::USER_VISIBLE, "default"},
+    {base::TaskPriority::USER_BLOCKING, "user_blocking"}};
+const base::Feature kSystemResolverPriorityExperiment = {
+    "SystemResolverPriorityExperiment", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::FeatureParam<base::TaskPriority> priority_mode{
+    &kSystemResolverPriorityExperiment, "mode",
+    base::TaskPriority::USER_VISIBLE, &prio_modes};
+
 //-----------------------------------------------------------------------------
 
 AddressList EnsurePortOnAddressList(const AddressList& list, uint16_t port) {
@@ -1980,7 +1990,8 @@ HostResolverImpl::HostResolverImpl(const Options& options, NetLog* net_log)
   DCHECK_GE(dispatcher_->num_priorities(), static_cast<size_t>(NUM_PRIORITIES));
 
   proc_task_runner_ = base::CreateTaskRunnerWithTraits(
-      {base::MayBlock(), base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN});
+      {base::MayBlock(), priority_mode.Get(),
+       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN});
 
 #if defined(OS_WIN)
   EnsureWinsockInit();