[Push] Add a type check for base::Value pref.

I'm not really sure why this is happening as we only store strings in
the dictionary. However there have been a few crash reports.

Bug: 1136031
Change-Id: I105bc90e4897ac01d3db5550382ac8b38e566347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470633Reviewed-by: Richard Knoll <knollr@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816996}

chrome/browser/push_messaging/push_messaging_app_identifier.cc

@@ -197,7 +197,8 @@ PushMessagingAppIdentifier PushMessagingAppIdentifier::FindByServiceWorker(
       profile->GetPrefs()->GetDictionary(prefs::kPushMessagingAppIdentifierMap);
   for (auto it = base::DictionaryValue::Iterator(*map); !it.IsAtEnd();
        it.Advance()) {
-    if (base::StartsWith(it.value().GetString(), base_pref_value,
+    if (it.value().is_string() &&
+        base::StartsWith(it.value().GetString(), base_pref_value,
                          base::CompareCase::SENSITIVE))
       return FindByAppId(profile, it.key());
   }