gn2bp: Add desc.json with target_cpu=x64

Decided to support x64 first, so update desc.json with target_cpu=x64

Used chromium at 14f5ebc46c254d2e4956f10c87e7ae7d8bc72969
Used following 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 = "x64"
```

Test: ./update_result.sh
Change-Id: I233ce489c2ee3ca047112ce8fe6ff2cf5d32f9d1