[chromium] Give the chromium enablement GN flag a more sensible name

This renames 'enable_chromium_based_web_runner' to just
'enable_chromium' since it now controls more than just the web_runner
implementation, and is a bit of a mouthful.

This leaves the previous spelling of the GN arg in place and defaults
it to the same as 'enable_chromium' to enable a soft transition with
code in other repositories that refers to this value.

Test:
US-472 #comment

Change-Id: Ib1c6735ed6a4a4e7b4744f0646e2840c7939a9cf
diff --git a/auth_providers/google/BUILD.gn b/auth_providers/google/BUILD.gn
index 70172fe..6b7e9d0 100644
--- a/auth_providers/google/BUILD.gn
+++ b/auth_providers/google/BUILD.gn
@@ -7,7 +7,7 @@
 import("//topaz/runtime/chromium/chromium.gni")
 
 config("config") {
-  if (enable_chromium_based_web_runner) {
+  if (enable_chromium) {
     defines = [ "DEFAULT_USE_CHROMIUM" ]
   }
 }
diff --git a/runtime/chromium/BUILD.gn b/runtime/chromium/BUILD.gn
index 4e45dfe..578e70e 100644
--- a/runtime/chromium/BUILD.gn
+++ b/runtime/chromium/BUILD.gn
@@ -17,7 +17,7 @@
   ]
 }
 
-if (enable_chromium_based_web_runner) {
+if (enable_chromium) {
   prebuilt_package("chromium") {
     archive = "//topaz/third_party/chromium/arch/$target_cpu/chromium.far"
   }
@@ -28,7 +28,7 @@
   }
 }
 
-if (enable_chromium_based_web_runner) {
+if (enable_chromium) {
   prebuilt_package("web_runner") {
     archive = "//topaz/third_party/chromium/arch/$target_cpu/web_runner.far"
   }
diff --git a/runtime/chromium/chromium.gni b/runtime/chromium/chromium.gni
index c164dd2..710062c 100644
--- a/runtime/chromium/chromium.gni
+++ b/runtime/chromium/chromium.gni
@@ -3,8 +3,12 @@
 # found in the LICENSE file.
 
 declare_args() {
+  enable_chromium_based_web_runner = false
+}
+
+declare_args() {
   # This enables a chromium-based web_runner implementation.
   # If this is set to false and web_runner_prototype is in the build, the
   # prototype version will be used instead to handle web requests.
-  enable_chromium_based_web_runner = false
+  enable_chromium = enable_chromium_based_web_runner
 }
diff --git a/runtime/web_runner_prototype/BUILD.gn b/runtime/web_runner_prototype/BUILD.gn
index e070606..578da65 100644
--- a/runtime/web_runner_prototype/BUILD.gn
+++ b/runtime/web_runner_prototype/BUILD.gn
@@ -5,7 +5,7 @@
 import("//build/package.gni")
 import("//topaz/runtime/chromium/chromium.gni")
 
-if (enable_chromium_based_web_runner) {
+if (enable_chromium) {
   package("web_runner_prototype") {
     deprecated_system_image = true
     # Empty
diff --git a/runtime/web_view/BUILD.gn b/runtime/web_view/BUILD.gn
index b767bc0..bfc70c0 100644
--- a/runtime/web_view/BUILD.gn
+++ b/runtime/web_view/BUILD.gn
@@ -6,7 +6,7 @@
 import("//topaz/runtime/chromium/chromium.gni")
 import("//topaz/runtime/web_view/config.gni")
 
-if (enable_chromium_based_web_runner) {
+if (enable_chromium) {
   # If chromium is the web runner, we don't need any of these targets.
   # Leave empty build & package targets for package configurations that still
   # reference these.