Fix Arc integration test.

components/arc/instance_holder.h

Index: components/arc/instance_holder.h
diff --git a/components/arc/instance_holder.h b/components/arc/instance_holder.h
index fda056e646139fb48e8dc8e2773598b2ebc8ae53..894a5a7f0fbc61fd460d3657d39801227b27b8cf 100644
--- a/components/arc/instance_holder.h
+++ b/components/arc/instance_holder.h
@@ -24,6 +24,10 @@
           holder)>::type::Instance::k##method_name##MinVersion, \
       #method_name)
 
+namespace {
+bool is_instance_for_sync_test = false;
+}  // namespace
+
 namespace arc {
 
 // Holds a Mojo instance+version pair. This also allows for listening for state
@@ -47,6 +51,12 @@ class InstanceHolder {
 
   using Instance = T;
 
+  // In sync integration test, mutiple instances are allowed to be created for
+  // different profiles.
+  static void SetInstanceHolderForSyncTest() {
+    is_instance_for_sync_test = true;
+  }
+
   InstanceHolder() = default;
 
   // Returns true if the Mojo interface is ready at least for its version 0
@@ -99,7 +109,8 @@ class InstanceHolder {
   // Passing nullptr to |instance| means closing.
   void SetInstance(T* instance, uint32_t version = T::Version_) {
     DCHECK(thread_checker_.CalledOnValidThread());
-    DCHECK(instance == nullptr || instance_ == nullptr);
+    DCHECK(instance == nullptr || instance_ == nullptr ||
+           is_instance_for_sync_test);
 
     // Note: This can be called with nullptr even if |instance_| is still
     // nullptr for just in case clean up purpose. No-op in such a case.