BUILD.gn: Make dawn_standalone default to false

BUG=chromium:870747

Change-Id: I9d80bb8a73227a9a9e5a7c466ac5b40a10fc0106
Reviewed-on: https://dawn-review.googlesource.com/1641
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

build_overrides/dawn.gni

@@ -17,7 +17,7 @@
 # Dawn's repository.
 
 # Whether we are building from Dawn's repository.
-# MUST be set to false in other projects.
+# MUST be unset in other projects (will default to false).
 dawn_standalone = true
 
 # Defaults for these are set again in dawn_overrides_with_defaults.gni so that

scripts/dawn_overrides_with_defaults.gni

@@ -20,6 +20,10 @@
 
 import("//build_overrides/dawn.gni")
 
+if (!defined(dawn_standalone)) {
+  dawn_standalone = false
+}
+
 if (!defined(dawn_spirv_cross_dir)) {
   dawn_spirv_cross_dir = "//third_party/spirv-cross"
 }