header_usage.sh script: Exclude matches in gyp and gn files.

NOTRY=true
BUG=None

Review-Url: https://codereview.webrtc.org/2363423002
Cr-Commit-Position: refs/heads/master@{#14382}
diff --git a/webrtc/tools/header_usage.sh b/webrtc/tools/header_usage.sh
index 6bb8901..58d8502 100755
--- a/webrtc/tools/header_usage.sh
+++ b/webrtc/tools/header_usage.sh
@@ -18,6 +18,8 @@
 
 git ls-files '*.h' '*.hpp' | while read header ; do
   name="$(basename "${header}")"
-  count="$(git grep -l -F  "${name}" | wc -l)"
+  count="$(git grep -l -F  "${name}" \
+           | grep -v -e '\.gn' -e '\.gyp'  \
+           | wc -l)"
   echo "${count}" "${header}"
 done | sort -n