Add helper script for running CTS APKs stored in the GS buckets.

android_webview/tools/cts_config/webview_cts_gcs_path.json

Index: android_webview/tools/cts_config/webview_cts_gcs_path.json
diff --git a/android_webview/tools/cts_config/webview_cts_gcs_path.json b/android_webview/tools/cts_config/webview_cts_gcs_path.json
index ad2250384476f674ae4aa5ba995b6d3ddf842149..5abb2fa3599ed560717a40eb9ad2a07feffa9b5a 100644
--- a/android_webview/tools/cts_config/webview_cts_gcs_path.json
+++ b/android_webview/tools/cts_config/webview_cts_gcs_path.json
@@ -2,14 +2,17 @@
   "arm_64": {
     "L": {
         "filename": "arm_64/L/android-cts-arm_64-3500656.zip",
+        "apk": "arm_64/L/3500656/CtsWebkitTestCases.apk",
         "_origin": "aosp-lollipop-mr1-cts-dev@3500656"
     },
     "M": {
         "filename": "arm_64/M/android-cts-arm_64-3505727.zip",
+        "apk": "arm_64/M/3505727/CtsWebkitTestCases.apk",
         "_origin": "aosp-marshmallow-cts-dev@3505727"
     },
     "N": {
         "filename": "arm_64/N/android-cts-arm_64-3367285.zip",
+        "apk": "arm_64/N/3367285/CtsWebkitTestCases.apk",
         "_origin": "aosp-nougat-cts-release@3367285"
     }
   }