Attempt to deflake UkmBrowserTest.RegularPlusIncognitoCheck

crrev.com/c/2340417 replaced a C++ method call with a mojo method call.
Apparently, the method, GetUserDataForAllRegistrations(), is called when
a profile is created on weblayer. It seems we need to wait for the
callback of GetUserDataForAllRegistrations() to be completed when a
profile is created or updated.

Bug: 1115172
Change-Id: I2b5f6614171b3502130127109bbe6822493f5497
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352268Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799383}

weblayer/browser/android/metrics/metrics_test_helper.cc

@@ -7,6 +7,7 @@
 #include "base/android/jni_android.h"
 #include "base/android/jni_string.h"
 #include "base/no_destructor.h"
+#include "base/run_loop.h"
 #include "weblayer/browser/profile_impl.h"
 #include "weblayer/test/weblayer_browsertests_jni/MetricsTestHelper_jni.h"
 
@@ -48,7 +49,11 @@ ProfileImpl* CreateProfile(const std::string& name) {
   JNIEnv* env = base::android::AttachCurrentThread();
   Java_MetricsTestHelper_createProfile(
       env, base::android::ConvertUTF8ToJavaString(env, name));
-  return GetProfileByName(name);
+  ProfileImpl* profile = GetProfileByName(name);
+  // Creating a profile may involve storage partition initialization. Wait for
+  // the initialization to be completed.
+  base::RunLoop().RunUntilIdle();
+  return profile;
 }
 void DestroyProfile(const std::string& name) {
   DCHECK(GetProfileByName(name));