gpu: Convert egl_main_native target to GN build.

Tested with the following command lines on Linux:

$ gn gen out-gn
$ ninja -C out-gn native
$ ninja -C out-gn gpu/gles2_conform_support/native
$ gn check out-gn //gpu/gles2_conform_support/native/*

BUG=432959
TEST=see above
R=dpranke@chromium.org,sievers@chromium.org

Review URL: https://codereview.chromium.org/992463002

Cr-Commit-Position: refs/heads/master@{#319765}

gpu/gles2_conform_support/gles2_conform_support.gyp

@@ -61,6 +61,7 @@
       'msvs_disabled_warnings': [ 4267, ],
     },
     {
+      # GN version: //gpu/gles2_conform_support/native
       'target_name': 'egl_main_native',
       'type': 'static_library',
       'dependencies': [

gpu/gles2_conform_support/native/BUILD.gn

+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# GYP version: gpu/gles2_conform_support/gles2_conform_support.gyp:egl_main_native
+source_set("native") {
+  output_name = "egl_main_native"
+  sources = [
+    "egl_native.cc",
+    "main.cc",
+  ]
+  defines = [
+    "GLES2_CONFORM_SUPPORT_ONLY",
+    "GTF_GLES20",
+    "EGLAPI=",
+    "EGLAPIENTRY=",
+  ]
+  deps = [
+    "//base",
+    "//gpu/gles2_conform_support/egl",
+  ]
+  if (is_linux) {
+    sources += [
+      "egl_native_aura.cc",
+      "egl_native_x11.cc",
+    ]
+  }
+  if (is_win) {
+    sources += [ "egl_native_win.cc" ]
+  }
+}