IDL compiler: Cache Jinja templates

Caching Jinja templates speeds up Python r-b-t from 14 seconds to 4 seconds (70% less time, 250% speed improvement)!

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/170503007

git-svn-id: svn://svn.chromium.org/blink/trunk@167402 bbb929c8-8fbe-4397-9dbb-9b2b20218538