Fix FirebaseMetadataProvider under VS2015 (#7188)

Author: wilhuff

Firestore/core/src/remote/firebase_metadata_provider.cc

@@ -20,9 +20,12 @@ namespace firebase {
 namespace firestore {
 namespace remote {
 
-constexpr char FirebaseMetadataProvider::kXFirebaseClientHeader[];
-constexpr char FirebaseMetadataProvider::kXFirebaseClientLogTypeHeader[];
-constexpr char FirebaseMetadataProvider::kXFirebaseGmpIdHeader[];
+const char FirebaseMetadataProvider::kXFirebaseClientHeader[] =
+    "x-firebase-client";
+const char FirebaseMetadataProvider::kXFirebaseClientLogTypeHeader[] =
+    "x-firebase-client-log-type";
+const char FirebaseMetadataProvider::kXFirebaseGmpIdHeader[] =
+    "x-firebase-gmpid";
 
 }  // namespace remote
 }  // namespace firestore

Firestore/core/src/remote/firebase_metadata_provider.h

@@ -31,10 +31,9 @@ namespace remote {
  */
 class FirebaseMetadataProvider {
  public:
-  static constexpr char kXFirebaseClientHeader[] = "x-firebase-client";
-  static constexpr char kXFirebaseClientLogTypeHeader[] =
-      "x-firebase-client-log-type";
-  static constexpr char kXFirebaseGmpIdHeader[] = "x-firebase-gmpid";
+  static const char kXFirebaseClientHeader[];
+  static const char kXFirebaseClientLogTypeHeader[];
+  static const char kXFirebaseGmpIdHeader[];
 
   virtual ~FirebaseMetadataProvider() = default;