emdawnwebgpu: add Symbol.iterator to externs

GPUSupportedFeatures and WGSLLanguageFeatures
implement Iterable but are missing Symbol.iterator in the Closure externs, which triggers warnings.

Bug: chromium:472022911
Test: emcc /tmp/webgpu_closure.c --closure 1 --closure-args="--externs=/tmp/dawn-externs.js --warning_level=VERBOSE --jscomp_warning=checkTypes" -O2 -o /tmp/webgpu_closure.js
Change-Id: Ie78a5d102ed0c132b10dc630bcc2a843de8e4acd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/282535
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>

Author: michaelgiraldo

third_party/emdawnwebgpu/pkg/webgpu/src/webgpu-externs.js

@@ -159,6 +159,8 @@ GPUSupportedFeatures.prototype.values = function(...args) {};
 GPUSupportedFeatures.prototype.forEach = function(...args) {};
 /** @return {boolean} */
 GPUSupportedFeatures.prototype.has = function(...args) {};
+/** @return {!Iterator<string>} */
+GPUSupportedFeatures.prototype[Symbol.iterator] = function() {};
 
 /**
  * @constructor
@@ -177,6 +179,8 @@ WGSLLanguageFeatures.prototype.values = function(...args) {};
 WGSLLanguageFeatures.prototype.forEach = function(...args) {};
 /** @return {boolean} */
 WGSLLanguageFeatures.prototype.has = function(...args) {};
+/** @return {!Iterator<string>} */
+WGSLLanguageFeatures.prototype[Symbol.iterator] = function() {};
 
 /** @constructor */
 function GPUAdapterInfo() {}