Avoid changing enabled features while the browser is already running.

base/feature_list.cc

Index: base/feature_list.cc
diff --git a/base/feature_list.cc b/base/feature_list.cc
index 61043ceb731fa7bf2c6891c67c95882121d5afa7..96b5af0e7568311ee04dc71c24f784836e6f68c7 100644
--- a/base/feature_list.cc
+++ b/base/feature_list.cc
@@ -25,6 +25,8 @@ namespace {
 // have more control over initialization timing. Leaky.
 FeatureList* g_instance = nullptr;
 
+bool g_block_overriding_instances_for_testing = false;
+
 // Tracks whether the FeatureList instance was initialized via an accessor.
 bool g_initialized_from_accessor = false;
 
@@ -280,6 +282,7 @@ void FeatureList::SetInstance(std::unique_ptr<FeatureList> instance) {
 
 // static
 std::unique_ptr<FeatureList> FeatureList::ClearInstanceForTesting() {
+  DCHECK(!g_block_overriding_instances_for_testing);
   FeatureList* old_instance = g_instance;
   g_instance = nullptr;
   g_initialized_from_accessor = false;
@@ -289,11 +292,17 @@ std::unique_ptr<FeatureList> FeatureList::ClearInstanceForTesting() {
 // static
 void FeatureList::RestoreInstanceForTesting(
     std::unique_ptr<FeatureList> instance) {
+  DCHECK(!g_block_overriding_instances_for_testing);
   DCHECK(!g_instance);
   // Note: Intentional leak of global singleton.
   g_instance = instance.release();
 }
 
+// static
+void FeatureList::BlockOverridingInstanceForTesting(bool block) {
+  g_block_overriding_instances_for_testing = block;
+}
+
 void FeatureList::FinalizeInitialization() {
   DCHECK(!initialized_);
   initialized_ = true;