Add a proper license to webgpu.h's template.

This avoids linter checks to trigger when importing updates of Dawn into
Google3.

Bug:
Change-Id: Ib038d8937cc6c294823b56d52b6f2c5e5e24ffac
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15140
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

generator/generator_lib.py

@@ -117,6 +117,11 @@ class _PreprocessingLoader(jinja2.BaseLoader):
         result = []
         indentation_level = 0
 
+        # Filter lines that are pure comments. line_comment_prefix is not enough because it removes
+        # the comment but doesn't completely remove the line, resulting in more verbose output.
+        lines = filter(lambda line: not line.strip().startswith('//*'), lines)
+
+        # Remove indentation templates have for the Jinja control flow.
         for line in lines:
             # The capture in the regex adds one element per block start or end so we divide by two
             # there is also an extra line chunk corresponding to the line end, so we substract it.

generator/templates/webgpu.h

@@ -1,3 +1,24 @@
+//* Copyright 2020 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.
+//*
+//*
+//* This template itself is part of the Dawn source and follows Dawn's license
+//* but the generated file is used for "WebGPU native". The template comments
+//* using //* at the top of the file are removed during generation such that
+//* the resulting file starts with the BSD 3-Clause comment.
+//*
+//*
 // BSD 3-Clause License
 //
 // Copyright (c) 2019, "WebGPU native" developers