[BACKPORT] Fix -Wstring-concatenation warnings in clang 12

Change-Id: Ic09346b6079e6f316c28e03ddb02f12b4af8a38d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/312482
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed