gn2bp: add desc_arm.json

This is a preparation to support multiple desc.json for differet
target_cpu.

desc_arm is generated with following configuration.

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

enable_base_tracing = false
target_cpu = "arm"
```
chromium at 14f5ebc46c254d2e4956f10c87e7ae7d8bc72969 with following
changes.
```
diff --git a/components/cronet/android/BUILD.gn b/components/cronet/android/BUILD.gn
index 16e3bbfd3f3..89d2f3d8a1e 100644
-- 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",
diff --git a/net/BUILD.gn b/net/BUILD.gn
index de630fff3c1..e380bd0dd3d 100644
-- a/net/BUILD.gn
++ b/net/BUILD.gn
@@ -1632,7 +1632,6 @@ source_set("net_deps") {
     ":constants",
     ":isolation_info_proto",
     ":net_export_header",
-    ":net_resources",
     ":preload_decoder",
     "//base",
     "//base/third_party/dynamic_annotations",
```

Test: ./gen_android_bp --desc desc_arm.json "//components/cronet/android:cronet"
Change-Id: Ifebde41e8ab39621b74cbf04ad8efee68d5bee39
diff --git a/tools/gn2bp/desc_arm.json b/tools/gn2bp/desc_arm.json
new file mode 100644
index 0000000..996b5f5
--- /dev/null
+++ b/tools/gn2bp/desc_arm.json
Binary files differ