protobuf: Add Message.FromString static method. (#3327)

Author: benjaminp

third_party/2and3/google/protobuf/message.pyi

@@ -1,4 +1,4 @@
-from typing import Any, Sequence, Optional, Tuple
+from typing import Any, Sequence, Optional, Tuple, Type, TypeVar
 
 from .descriptor import (
     DescriptorBase,
@@ -13,6 +13,8 @@ class _ExtensionDict:
     def __getitem__(self, extension_handle: DescriptorBase) -> Any: ...
     def __setitem__(self, extension_handle: DescriptorBase, value: Any) -> None: ...
 
+_T = TypeVar("_T")
+
 class Message:
     DESCRIPTOR: Any
     def __deepcopy__(self, memo=...): ...
@@ -31,6 +33,8 @@ class Message:
     def HasExtension(self, extension_handle): ...
     def ClearExtension(self, extension_handle): ...
     def ByteSize(self) -> int: ...
+    @classmethod
+    def FromString(cls: Type[_T], s: Any) -> _T: ...
     @property
     def Extensions(self) -> _ExtensionDict: ...