Standardize strings and other cleanup in bloaty_treemap.py

Follow-up from reviews.skia.org/428959

Change-Id: I3a9baaf1744dcfcea372c5a0c73af31912888c0c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/429818
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed