This updates the comments and code regarding GOMA_DISABLED to more
precisely match its behavior and therefore avoid confusion.

Bug: b/140312943
Change-Id: I6208de282bd5ebf1d88f98c1f905cfc626de792e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1786676
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>