Move build_with_chromium GN variable to DEPS file.

This way, DEPS files can also know whether they're building with chromium
or not.

Bug: 782846
Change-Id: I43885e2b9aa1e72b612c17569a23d47e74835273
Reviewed-on: https://chromium-review.googlesource.com/1077619Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563160}

DEPS

@@ -28,6 +28,7 @@
 
 gclient_gn_args_file = 'src/build/config/gclient_args.gni'
 gclient_gn_args = [
+  'build_with_chromium',
   'checkout_android',
   'checkout_android_native_support',
   'checkout_libaom',
@@ -37,6 +38,11 @@ gclient_gn_args = [
 
 
 vars = {
+  # Variable that can be used to support multiple build scenarios, like having
+  # Chromium specific targets in a client project's GN file or sync dependencies
+  # conditionally etc.
+  'build_with_chromium': True,
+
   # By default, we should check out everything needed to run on the main
   # chromium waterfalls. This var can be also be set to "small", in order
   # to skip things are not strictly needed to build chromium for development

build_overrides/build.gni

@@ -2,9 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-# Variable that can be used to support multiple build scenarios, like having
-# Chromium specific targets in a client project's GN file etc.
-build_with_chromium = true
+import("//build/config/gclient_args.gni")
 
 # Uncomment these to specify a different NDK location and version in
 # non-Chromium builds.