Tag flags that only apply to certain environments

[beverloo]

Some command line flags in Chrome are not available for the entirety of the project, but rather only in specific environments, e.g. Mojo. Take the following flag:

https://code.google.com/p/chromium/codesearch#chromium/src/mojo/runner/switches.cc&q=kUseTemporaryUserDataDir&sq=package:chromium&type=cs&l=49

This is being presented as follows:

http://peter.sh/experiments/chromium-command-line-switches/#use-temporary-user-data-dir

.. yet is only available in the Mojo runner. A tag would significantly help in clarifying this.

[Context: https://crbug.com/529837]