[infra] Add back valgrind dimension for Valgrind tasks

In https://skia-review.googlesource.com/c/skia/+/191741, I was hopeful
that this wouldn't be necessary, but it is.

Change-Id: I8904d5c324372e6623431db781a7a4c67a18641c
Reviewed-on: https://skia-review.googlesource.com/c/193174
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed