Split dawn_(native|wire|platform) to their own BUILD.gn files

This will help external projects that want to use Dawn, like Skia, refer
to GN targets without GN discovering Dawn's main BUILD.gn file that
causes all the tests and third_party dependencies to be discovered.

This CL just splits off chunks of BUILD.gn into separate file, adds
necessary includes and fixes up GN paths. It also introduces temporary
groups for targets that are used in Chromium so that the paths can be
fixed in a 3-way patch.

Bug: chromium:1064305

Change-Id: Ib4b73bd8d3121ef67d4ecee2e54ec158875f2117
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18861
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_platform/BUILD.gn

@@ -0,0 +1,30 @@
+# 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.
+
+import("../../scripts/dawn_overrides_with_defaults.gni")
+
+source_set("dawn_platform") {
+  configs += [ "${dawn_root}/src/common:dawn_internal" ]
+
+  sources = [
+    "${dawn_root}/src/include/dawn_platform/DawnPlatform.h",
+    "tracing/EventTracer.cpp",
+    "tracing/EventTracer.h",
+    "tracing/TraceEvent.h",
+  ]
+
+  deps = [
+    "${dawn_root}/src/common",
+  ]
+}