Allow override of "checkout_google_benchmark" via build_overrides.

WebRTC doesn't use gclient_args.gni, this CL adds the possibility to
override the value of "checkout_google_benchmark" with another
override method (//build_overrides/build.gni).

Bug: None
Change-Id: If6e6cd9b9cbeb82d96e7c5b578347884e0c84607
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218024
Reviewed-by: Markus Handell <handellm@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773129}
NOKEYCHECK=True
GitOrigin-RevId: 5bfaa53575c1a0948cbbc2332bcb3619df6584a9