Fetch clang-format automatically when compiling .fp files.

On a freshly fetched repo, setting `skia_compile_processors = true` will
fail to compile because clang-format is missing from the bin directory.
This CL automatically runs fetch-clang-format for you when clang-format
is absent.

Change-Id: Ieeb359176072e92ca235316c820310333732f608
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295780
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed