build: Override angle_has_rapidjson to false

This fixes the issue where a standalone checkout of Dawn could not build
the |all| target because ANGLE was missing sub-dependencies. The
|:default| target still builds fine before this change.

Change-Id: Icf6582068837053909d9efd1cffd3a0d3c0836cf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/61380
Commit-Queue: Michael Tang <tangm@microsoft.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

.gn

diff --git a/.gn b/.gn
index 2b8936e..84cb715 100644
--- a/.gn
+++ b/.gn
@@ -27,6 +27,7 @@
   angle_enable_abseil = false
   angle_standalone = false
   angle_build_all = false
+  angle_has_rapidjson = false
   angle_vulkan_headers_dir = "//third_party/vulkan-deps/vulkan-headers/src"
   angle_vulkan_loader_dir = "//third_party/vulkan-deps/vulkan-loader/src"
   angle_vulkan_tools_dir = "//third_party/vulkan-deps/vulkan-tools/src"