Add GenerateClassFromObject to Javascript gen

Adds the fromObject helper to the public API of generated pb classes.

This closes a 5-year-old open issue: https://github.com/protocolbuffers/protobuf/issues/1591 
by rebasing this 1-year-old open PR: protocolbuffers/protobuf#8488
based on this 4-year-old workaround: https://github.com/protocolbuffers/protobuf/issues/1591#issuecomment-414778829

generator/js_generator.cc

@@ -1978,6 +1978,8 @@ void Generator::GenerateClass(const GeneratorOptions& options,
     GenerateClassFieldInfo(options, printer, desc);
 
     GenerateClassToObject(options, printer, desc);
+    GenerateClassFromObject(options, printer, desc);
+    printer->Print("\ngoog.isDef = goog.isDef || function(a) { if (a === undefined || a === null) { return false; }; return true; }; goog.array = { map: function(arr, func) { return arr.map(func); }; }\n");
     // These must come *before* the extension-field info generation in
     // GenerateClassRegistration so that references to the binary
     // serialization/deserialization functions may be placed in the extension