gn2bp: Add desc.json to generate .bp for targets

desc.json is generated by
```
$ gn gen out/test
$ cp args.gn out/test/args.gn
$ gn desc out/test --format=json --all-toolchains "//*" > desc.json
```
chromium commit hash is 14f5ebc46c254d2e4956f10c87e7ae7d8bc72969

deps for cronet_package_android is
```
diff --git a/components/cronet/android/BUILD.gn b/components/cronet/android/BUILD.gn
index 16e3bbfd3f3..89d2f3d8a1e 100644

Change-Id: I618650a0be60539064bee163b193aa137c7c0d4f

--- a/components/cronet/android/BUILD.gn
+++ b/components/cronet/android/BUILD.gn
@@ -1600,22 +1600,8 @@ if (!is_component_build) {
     if (use_platform_icu_alternatives &&
         (!(target_cpu == "arm" && arm_version == 7) || !arm_use_neon)) {
       deps = [
-        ":api_static_checks",
-        ":cronet_package_copy",
-        ":cronet_package_copy_native_headers",
         ":cronet_package_copy_native_lib",
-        ":cronet_package_copy_native_lib_unstripped",
-        ":cronet_package_copy_resources",
-        ":cronet_sizes",
-        ":cronet_test_package",
-        ":generate_javadoc",
-        ":generate_licenses",
         ":jar_cronet_api_source",
-        ":jar_cronet_impl_common_java_source",
-        ":jar_cronet_impl_fake_java_source",
-        ":jar_cronet_impl_native_java_source",
-        ":jar_cronet_impl_platform_java_source",
-        ":jar_cronet_sample_source",
         ":repackage_api",
         ":repackage_common",
         ":repackage_fake",
```

args.gn is
```
$ cat out/test/args.gn
target_os = "android"
enable_websockets = false
disable_file_support = true
disable_brotli_filter = false
is_component_build = false
use_crash_key_stubs = true
use_partition_alloc = false
include_transport_security_state_preload_list = false
use_platform_icu_alternatives = true
default_min_sdk_version = 19
use_errorprone_java_compiler = true
enable_reporting = true
use_hashed_jni_names = true
treat_warnings_as_errors = false
arm_use_neon = false

enable_base_tracing = false
```

Test: ./update_results.sh
Change-Id: I0ae2d70e05c6c6f86e9986ab24e7df18b3a918e8