IDL compiler: switch build to Python compiler

This switches the build to Python, rather than Perl.

As of Blink r-167947, Perl and Python generate identical bindings code, so we're ready to switch:
https://src.chromium.org/viewvc/blink?view=rev&revision=167947

Certain in-flight CLs may cause breakage if they land:
if they only change Perl (not Python).
This is generally caught by run-bindings-tests in blink_presubmit, but can fail if the change lacks test cases,
or if the patch set was uploaded prior to turning on Python tests in r-167838.

The only 1 I can see is:
Move mediastream module to oilpan
https://codereview.chromium.org/173363002/
This lacks test cases, and will break the build if it lands.
I've stopped this.

Ref:
log: code_generator_v8.pm
http://src.chromium.org/viewvc/blink/trunk/Source/bindings/scripts/code_generator_v8.pm?view=log#revHEAD

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/166333002

git-svn-id: svn://svn.chromium.org/blink/trunk@167983 bbb929c8-8fbe-4397-9dbb-9b2b20218538