Update gen_keycodes output to new engine location. (#162479)

The engine is now part of Flutter and doesn't need to have the path
configured.

The current code doesn't update the engine.

Author: robert-ancell

dev/tools/gen_keycodes/README.md

@@ -38,10 +38,6 @@ anew before generating the files. To do this, run:
 This will generate `physical_key_data.g.json` and `logical_key_data.g.json`. These
 files should be checked in.
 
-By default this tool assumes that the gclient directory for flutter/engine
-and the root for the flutter/flutter are placed at the same folder. If not,
-use `--engine-root=/ENGINE/GCLIENT/ROOT` to specify the engine root.
-
 Other options can be found using `--help`.
 
 ## Key ID Scheme

dev/tools/gen_keycodes/bin/gen_keycodes.dart

@@ -111,16 +111,6 @@ Future<void> main(List<String> rawArguments) async {
     return;
   }
   final ArgParser argParser = ArgParser();
-  argParser.addOption(
-    'engine-root',
-    defaultsTo: path.join(flutterRoot.path, '..', 'engine', 'src', 'flutter'),
-    help:
-        'The path to the root of the flutter/engine repository. This is used '
-        'to place the generated engine mapping files. If --engine-root is not '
-        r'specified, it will default to $flutterRoot/../engine/src/flutter, '
-        'assuming the engine gclient folder is placed at the same folder as '
-        'the flutter/flutter repository.',
-  );
   argParser.addOption(
     'physical-data',
     defaultsTo: path.join(dataRoot, 'physical_key_data.g.json'),
@@ -193,7 +183,7 @@ Future<void> main(List<String> rawArguments) async {
     exit(0);
   }
 
-  PlatformCodeGenerator.engineRoot = parsedArguments['engine-root'] as String;
+  PlatformCodeGenerator.engineRoot = path.join(flutterRoot.path, 'engine', 'src', 'flutter');
 
   PhysicalKeyData physicalData;
   LogicalKeyData logicalData;