PRESUBMIT.py: Add formatting checks.

GN files are checked using the canned presubmit check but we cannot do
the same for C/C++ files: the canned check uses git cl format which can
only use clang-format 5.0 and requires formatting of all the source
code.

Instead we write our own clang-format check, reusing the script that
checks formatting on Travis. To have a recent version of clang-format we
import one from a helper repo.

This also fix a formatting error in .gn and adds licenses to the
clang-format linting scripts.

Change-Id: I4d8208472a8a6bd32ae3ef41c3145abf270a4c37

scripts/lint_clang_format.sh

@@ -0,0 +1,44 @@
+#!/bin/bash
+
+# Copyright 2018 The Dawn Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+clang_format=$1
+base_commit=$2
+
+echo
+skipped_directories="(examples|generator|src/tests/(unittests|end2end)|third_party)"
+# Find the files modified that need formatting
+files_to_check=$(git diff --diff-filter=ACMR --name-only $base_commit | grep -E "*\.(c|cpp|mm|h)$" | grep -vE "^$skipped_directories/*")
+if [ -z "$files_to_check" ]; then
+    echo "No modified files to format."
+    exit 0
+fi
+echo "Checking formatting diff on these files:"
+echo "$files_to_check"
+echo
+files_to_check=$(echo $files_to_check | tr '\n' ' ')
+
+# Run git-clang-format, check if it formatted anything
+format_output=$(scripts/git-clang-format --binary $clang_format --commit $base_commit --diff --style=file $files_to_check)
+if [ "$format_output" == "clang-format did not modify any files" ] || [ "$format_output" == "no modified files to format" ] ; then
+    exit 0
+fi
+
+# clang-format made changes, print them and fail Travis
+echo "Following formatting changes needed:"
+echo
+echo "$format_output"
+echo
+exit 1