cronet import: output gn desc files in correct location

Writing changes directly to android/tools/gn2bp makes the process of
updating the gn desc files a bit easier.

Test: run script
Change-Id: Ie75872f1640e06ac880d7b0fcf5cb23e5537ea51
diff --git a/android/tools/gn2bp/gen_desc_json.sh b/android/tools/gn2bp/gen_desc_json.sh
index bac3601..01cd2af 100755
--- a/android/tools/gn2bp/gen_desc_json.sh
+++ b/android/tools/gn2bp/gen_desc_json.sh
@@ -96,6 +96,7 @@
 #######################################
 # Generate desc.json for a specified architecture.
 # Globals:
+#   ANDROID_BUILD_TOP
 #   OUT_PATH
 # Arguments:
 #   target_cpu, string
@@ -143,7 +144,7 @@
   gn gen "${OUT_PATH}" --args="${gn_args[*]}"
 
   # Generate desc.json.
-  local -r out_file="desc_${target_cpu}.json"
+  local -r out_file="${ANDROID_BUILD_TOP}/external/cronet/android/tools/gn2bp/desc_${target_cpu}.json"
   gn desc "${OUT_PATH}" --format=json --all-toolchains "//*" > "${out_file}"
 )