fixup for builder commit

scalapb-runtime/shared/src/main/scala/scalapb/GeneratedMessageCompanion.scala

@@ -154,20 +154,16 @@ trait JavaProtoSupport[ScalaPB, JavaPB] extends Any {
 trait GeneratedMessageCompanion[A <: GeneratedMessage] {
   type ValueType = A
 
-  type Builder <: MessageBuilder[A]
-
   def merge(a: A, input: CodedInputStream): A
 
-  def parseFrom(input: CodedInputStream): A = newBuilder.merge(input).result
+  /** Parses a message from a CodedInputStream. */
+  // TODO(nadav): The implementation here should not rely on `newBuilder` for binary compatibility:
+  // Older 0.10.x generated code may not have it defined. In 0.10.x, the generated messages override
+  // this method. This can be eliminated in ScalaPB 0.11.0.
+  def parseFrom(input: CodedInputStream): A = merge(defaultInstance, input)
 
   def parseFrom(input: InputStream): A = parseFrom(CodedInputStream.newInstance(input))
 
-  /** Returns a new empty Builder that can be used to build instances of this message */
-  def newBuilder: Builder
-
-  /** Returns a new Builder initialized from the given message */
-  def newBuilder(a: A): Builder
-
   def parseDelimitedFrom(input: CodedInputStream): Option[A] =
     LiteParser.parseDelimitedFrom(input)(this)