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/DEPS b/DEPS
index d93ea08..0694795 100644
--- a/DEPS
+++ b/DEPS
@@ -87,7 +87,7 @@
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     'pattern': '.',
-    'action': ['python', 'src/webrtc/build/gyp_webrtc',
+    'action': ['python', 'src/webrtc/build/gyp_webrtc.py',
                Var('extra_gyp_flag')],
   },
 ]