Style guide: Add sections for Java, Objective-C/C++, and Python

They're just pointers to the corresponding Chromium or Google style
guides.

BUG=none
NOTRY=true

Change-Id: Ib808db1cc4fc4d8a4ea708e6ec1c92d6d219b78e
Reviewed-on: https://chromium-review.googlesource.com/657419
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Kári Tristan Helgason <kthelgason@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Niels Möller <nisse@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Cr-Commit-Position: refs/heads/master@{#19758}