PSA: change in default behavior of `content_shell --run-web-tests`

[Stefan Zager]

If you never run `content_shell --run-web-tests` from the command line, you can stop reading now.

tl;dr -- `content_shell --run-web-tests` will soon switch to using threaded compositing by default.

Currently, the default behavior of `content_shell --run-web-tests` is to disable threaded compositing and threaded animations. As part of an effort to enable threaded compositing by default for (web|wpt)_tests, we will invert that behavior, i.e., the default behavior will be to run with threaded compositing and threaded animations.

This will *not* change any behavior when using the test runner scripts (run_web_tests.py and run_wpt_tests.py). Those scripts always explicitly specify their behavior by adding either `--enable-threaded-compositing` or `--disable-threaded-compositing --disable-threaded-animation` to the command line for content_shell. There is only a change in behavior if you run `content_shell --run-web-tests` without an explicit `--(en|dis)able_threaded-compositing` argument.

Questions/comments/suggestions: yotha@ and/or szager@