Fix assert message in grit build

The "extra" and "missing" lists in the output file assert message are swapped

This is how it should be:
Extra = files that are actually there but I did not expect
      = files that are left when I take what I actually have and remove what I expected
      = actual - expected

Missing = files that are expected but are not actually there
        = files that are left when I take what I expected and remove what I actually have
        = expected - actual

Change-Id: Iea5ad5a6f3718e937e14e2fa0e1876e30530c3a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525984
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#641248}
1 file changed