Add support for generating custom messages

Summary:
Until now we've generated scaffolding entirely based on the official devtools
protocol spec. This diff adds support for defining custom domains in `custom.json`
which will be merged with the upstream protocol JSON definition.

ChangeLog: [Internal] Add support for Hermes-specific CDP messages

Reviewed By: bestander

Differential Revision: D20754605

fbshipit-source-id: a8075f81816a40114d1a3332192c7aa076b17848

ReactCommon/hermes/inspector/tools/msggen/src/custom.json

@@ -0,0 +1,3 @@
+{
+  "domains": []
+}

ReactCommon/hermes/inspector/tools/msggen/src/index.js

@@ -24,7 +24,10 @@ import {HeaderWriter} from './HeaderWriter';
 import {ImplementationWriter} from './ImplementationWriter';
 
 // $FlowFixMe: this isn't a module, just a JSON file.
-const proto = require('devtools-protocol/json/js_protocol.json');
+const standard = require('devtools-protocol/json/js_protocol.json');
+
+// $FlowFixMe: this isn't a module, also just a JSON file.
+const custom = require('../src/custom.json');
 
 type Descriptor = {|
   types: Array<Type>,
@@ -32,6 +35,12 @@ type Descriptor = {|
   events: Array<Event>,
 |};
 
+function mergeDomains(original, extra) {
+  return {...original, domains: original.domains.concat(extra.domains)};
+}
+
+const proto = mergeDomains(standard, custom);
+
 function parseDomains(
   domainObjs: Array<any>,
   ignoreExperimental: boolean,