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>
diff --git a/gn/compile_processors.py b/gn/compile_processors.py
index 922b653..1ffc1ae 100755
--- a/gn/compile_processors.py
+++ b/gn/compile_processors.py
@@ -11,14 +11,19 @@
 
 skslc = sys.argv[1]
 clangFormat = sys.argv[2]
-processors = sys.argv[3:]
+fetchClangFormat = sys.argv[3]
+processors = sys.argv[4:]
+
 for p in processors:
     print("Recompiling " + p + "...")
     try:
+        if not os.path.isfile(clangFormat):
+            subprocess.check_call(fetchClangFormat)
+
         noExt, _ = os.path.splitext(p)
         head, tail = os.path.split(noExt)
         targetDir = os.path.join(head, "generated")
-        if not os.path.exists(targetDir):
+        if not os.path.isdir(targetDir):
             os.mkdir(targetDir)
         target = os.path.join(targetDir, tail)
         subprocess.check_output([skslc, p, target + ".h"])