[Jobs API]: Implement CancelAndDetach in gin.

Change-Id: I5a837c6e49c0beec1db69a54d6c8653e7d688045
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459348
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815114}

gin/v8_platform.cc

@@ -312,6 +312,7 @@ class JobHandleImpl : public v8::JobHandle {
   }
   void Join() override { handle_.Join(); }
   void Cancel() override { handle_.Cancel(); }
+  void CancelAndDetach() override { handle_.CancelAndDetach(); }
   bool IsCompleted() override { return handle_.IsCompleted(); }
   bool IsRunning() override { return !!handle_; }