Add *.proto to git-gs

Change-Id: I393579c9cfafbd6a5331abb13e33b61c8fa69df7
Reviewed-on: https://chromium-review.googlesource.com/473548
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

git-gs

@@ -6,4 +6,4 @@ git grep -n -e "$@" -- "*.h" "*.hpp" "*.cpp" "*.c" "*.cc" "*.cpp" "*.inl"\
    "*.grd" "*.grdp" "*.idl" "*.m" "*.mm" "*.py" "*.sh" "*.cfg" "*.tac" "*.go"\
    "*.vcproj" "*.vsprops" "*.make" "*.gyp" "*.gypi" "*.isolate" "*.java"\
    "*.js" "*.html" "*.css" "*.ebuild" "*.pl" "*.pm" "*.yaml" "*.gn" "*.gni"\
-   "*.json" "DEPS" "*/DEPS" "*.mojom"
+   "*.json" "DEPS" "*/DEPS" "*.mojom" "*.proto"