The build summary script was overly aggressive about summarizing .mojom
files under the mojo category. This was intended to measure the cost of
generating source from mojo files but it accidentally pulled in the
.mojom object files.

Categorizing different types of object files might be worthwhile but
that should be done intentionally and consistently.

Change-Id: Iab6b7e94797ce7f1ed46805034b4f274c88617e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2067647
Commit-Queue: Aaron Gable <agable@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>