Simplify Serializer [En|De]code*() methods to no longer [de]serialize… (#2013)

Now, we use the re-worked nanopb deserializer (that mallocs). This also
had the advantage of moving the C++ serialization code closer to the
other platforms.

Still TODO:
- Reorder the methods within the files to resemble the other platforms.

Author: rsgowman

FirebaseFirestore.podspec

@@ -57,7 +57,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
   s.dependency 'gRPC-C++', '0.0.5'
   s.dependency 'leveldb-library', '~> 1.20'
   s.dependency 'Protobuf', '~> 3.1'
-  s.dependency 'nanopb', '~> 0.3.8'
+  s.dependency 'nanopb', '~> 0.3.901'
 
   s.frameworks = 'MobileCoreServices', 'SystemConfiguration'
   s.library = 'c++'

Firestore/Example/Firestore.xcodeproj/project.pbxproj

@@ -2492,6 +2492,7 @@
 					"\"${PODS_ROOT}/GoogleTest/googlemock/include\"",
 					"\"${PODS_ROOT}/GoogleTest/googletest/include\"",
 					"\"${PODS_ROOT}/leveldb-library/include\"",
+					"\"${PODS_ROOT}/../../../Firestore/Protos/nanopb\"",
 					"\"${PODS_ROOT}/../../../Firestore/Protos/cpp\"",
 					"\"${PODS_ROOT}/ProtobufCpp/src\"",
 				);
@@ -2569,6 +2570,7 @@
 					"\"${PODS_ROOT}/GoogleTest/googlemock/include\"",
 					"\"${PODS_ROOT}/GoogleTest/googletest/include\"",
 					"\"${PODS_ROOT}/leveldb-library/include\"",
+					"\"${PODS_ROOT}/../../../Firestore/Protos/nanopb\"",
 					"\"${PODS_ROOT}/../../../Firestore/Protos/cpp\"",
 					"\"${PODS_ROOT}/ProtobufCpp/src\"",
 				);
@@ -2640,6 +2642,7 @@
 					"\"${PODS_ROOT}/../../..\"",
 					"\"${PODS_ROOT}/../../../Firestore/third_party/abseil-cpp\"",
 					"\"${PODS_ROOT}/nanopb\"",
+					"\"${PODS_ROOT}/../../../Firestore/Protos/nanopb\"",
 				);
 				INFOPLIST_FILE = "FuzzTests/Firestore_FuzzTests_iOS-Info.plist";
 				OTHER_CFLAGS = (
@@ -2672,6 +2675,7 @@
 					"\"${PODS_ROOT}/../../..\"",
 					"\"${PODS_ROOT}/../../../Firestore/third_party/abseil-cpp\"",
 					"\"${PODS_ROOT}/nanopb\"",
+					"\"${PODS_ROOT}/../../../Firestore/Protos/nanopb\"",
 				);
 				INFOPLIST_FILE = "FuzzTests/Firestore_FuzzTests_iOS-Info.plist";
 				OTHER_CFLAGS = (

Firestore/Example/FuzzTests/FuzzingTargets/FSTFuzzTestSerializer.mm

@@ -38,7 +38,9 @@ int FuzzTestDeserialization(const uint8_t *data, size_t size) {
   @autoreleasepool {
     @try {
       Reader reader = Reader::Wrap(data, size);
-      serializer.DecodeFieldValue(&reader);
+      google_firestore_v1_Value nanopb_proto{};
+      reader.ReadNanopbMessage(google_firestore_v1_Value_fields, &nanopb_proto);
+      serializer.DecodeFieldValue(&reader, nanopb_proto);
     } @catch (...) {
       // Caught exceptions are ignored because the input might be malformed and
       // the deserialization might throw an error as intended. Fuzzing focuses on