Move logic of gyp_webrtc into gyp_webrtc.py

For historical reasons gyp_webrtc.py was launcher script
for gyp_webrtc and the python logic lived in the
gyp_webrtc.  This change moves python code into the
.py file makes the extension-free gyp_webrtc a launcher
for gyp_webrtc.py.

Other changes:
* Move the code into a main() function.
* Add call to disable GC to save some processing time.
* Set executable permission on gyp_webrtc.py and remove it from
  gyp_webrtc.

Similar Chromium CL: https://codereview.chromium.org/1216863010

Motivation for this change:
* Gets checked with PyLint
* Easy to add unit tests if we add our own functionality.

R=phoglund@webrtc.org
TBR=tkchin@webrtc.org

Review URL: https://codereview.webrtc.org/1895713002 .

Cr-Commit-Position: refs/heads/master@{#12410}
diff --git a/webrtc/build/ios/build_ios_libs.sh b/webrtc/build/ios/build_ios_libs.sh
index a01d128..5617552 100755
--- a/webrtc/build/ios/build_ios_libs.sh
+++ b/webrtc/build/ios/build_ios_libs.sh
@@ -16,7 +16,7 @@
 # Globals.
 SCRIPT_DIR=$(cd $(dirname $0) && pwd)
 WEBRTC_BASE_DIR=${SCRIPT_DIR}/../../..
-GYP_WEBRTC_SCRIPT=${WEBRTC_BASE_DIR}/webrtc/build/gyp_webrtc
+GYP_WEBRTC_SCRIPT=${WEBRTC_BASE_DIR}/webrtc/build/gyp_webrtc.py
 EXPORT_HEADERS_SCRIPT=${SCRIPT_DIR}/export_headers.py
 MERGE_SCRIPT=${SCRIPT_DIR}/merge_ios_libs.py
 
@@ -70,7 +70,7 @@
 
   # GYP generation requires relative path for some reason.
   pushd ${WEBRTC_BASE_DIR}
-  webrtc/build/gyp_webrtc webrtc/build/ios/merge_ios_libs.gyp
+  ${GYP_WEBRTC_SCRIPT} webrtc/build/ios/merge_ios_libs.gyp
   popd
   if [[ ${USE_LEGACY_API} -eq 1 ]]; then
     ninja -C ${ninja_output_dir}/${flavor} libjingle_peerconnection_objc_no_op