Faster run-bindings-tests

This makes run-bindings-tests faster by doing more work in
Python, instead of going out to the OS.
Notably: calling Python compiler directly (reusing a compiler object),
instead of launching separate processes, and
avoiding a temporary file for interfaces_info.

(Earlier versions had many refactoring changes,
which improve support for Dart CG and other backends,
which Nils broke off and committed separately.)

R=nbarth

Review URL: https://codereview.chromium.org/169743005

git-svn-id: svn://svn.chromium.org/blink/trunk@169440 bbb929c8-8fbe-4397-9dbb-9b2b20218538