Have git gs search *.gn too.

BUG=None

Review URL: https://codereview.chromium.org/126923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@243431 0039d316-1c4b-4281-b951-d872f2087c98

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"\
    "*SConscript" "SConscript*" "*.scons" "*.vcproj" "*.vsprops" "*.make"\
    "*.gyp" "*.gypi" "*.isolate" "*.java" "*.js" "*.html" "*.css" "*.ebuild" \
-   "*.pl" "*.pm" "*.yaml"
+   "*.pl" "*.pm" "*.yaml" "*.gn"