Efficient Conversion of a FlatBufferBuilder to a MessageBuilder (#4980)

* Efficient conversion of FlatBufferBuilder to grpc::MessageBuilder

* Added a variety of tests to validate correctness of the MessageBuilder move operations.
Disable MessageBuilder half-n-half tests on MacOS.

* Fix failing Android build

* Generalized the MessageBuilder move constructor to accept a deallocator

include/flatbuffers/grpc.h

@@ -164,7 +164,7 @@ class MessageBuilder : private detail::SliceAllocatorMember,
                        public FlatBufferBuilder {
  public:
   explicit MessageBuilder(uoffset_t initial_size = 1024)
-      : FlatBufferBuilder(initial_size, &slice_allocator_, false) {}
+    : FlatBufferBuilder(initial_size, &slice_allocator_, false) {}
 
   MessageBuilder(const MessageBuilder &other) = delete;
   MessageBuilder &operator=(const MessageBuilder &other) = delete;
@@ -175,6 +175,30 @@ class MessageBuilder : private detail::SliceAllocatorMember,
     Swap(other);
   }
 
+  /// Create a MessageBuilder from a FlatBufferBuilder.
+  explicit MessageBuilder(FlatBufferBuilder &&src, void (*dealloc)(void*, size_t) = &DefaultAllocator::dealloc)
+    : FlatBufferBuilder(1024, &slice_allocator_, false) {
+    src.Swap(*this);
+    src.SwapBufAllocator(*this);
+    if (buf_.capacity()) {
+      uint8_t *buf = buf_.scratch_data();       // pointer to memory
+      size_t capacity = buf_.capacity();        // size of memory
+      slice_allocator_.slice_ = grpc_slice_new_with_len(buf, capacity, dealloc);
+    }
+    else {
+      slice_allocator_.slice_ = grpc_empty_slice();
+    }
+  }
+
+  /// Move-assign a FlatBufferBuilder to a MessageBuilder.
+  /// Only FlatBufferBuilder with default allocator (basically, nullptr) is supported.
+  MessageBuilder &operator=(FlatBufferBuilder &&src) {
+    // Move construct a temporary and swap
+    MessageBuilder temp(std::move(src));
+    Swap(temp);
+    return *this;
+  }
+
   MessageBuilder &operator=(MessageBuilder &&other) {
     // Move construct a temporary and swap
     MessageBuilder temp(std::move(other));