build: define compile time features with buildflag header (#14840)

* build: define compile time features with buildflag header

* refactor: switch to BUILDFLAG(ENABLE_DESKTOP_CAPTURER)

* refactor: switch to BUILDFLAG(ENABLE_RUN_AS_NODE)

* refactor: switch to BUILDFLAG(ENABLE_OSR)

* refactor: switch to BUILDFLAG(ENABLE_VIEW_API)

* refactor: switch to BUILDFLAG(ENABLE_PEPPER_FLASH)

* refactor: switch to BUILDFLAG(OVERRIDE_LOCATION_PROVIDER)

* refactor: switch to BUILDFLAG(ENABLE_PDF_VIEWER)

buildflags/buildflags.gni

@@ -0,0 +1,25 @@
+# Copyright (c) 2018 GitHub, Inc.
+# Use of this source code is governed by the MIT license that can be
+# found in the LICENSE file.
+
+declare_args() {
+  enable_desktop_capturer = true
+
+  # Allow running Electron as a node binary.
+  enable_run_as_node = true
+
+  enable_osr = true
+
+  enable_view_api = false
+
+  enable_pdf_viewer = false
+
+  # Provide a fake location provider for mocking
+  # the geolocation responses. Disable it if you
+  # need to test with chromium's location provider.
+  # Should not be enabled for release build.
+  enable_fake_location_provider = !is_official_build
+
+  # Enable flash plugin support.
+  enable_pepper_flash = true
+}