Currently, the "ios" recipe module generates a single isolate template with variables, which are then instantiated per task: https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/ios/api.py?l=750&rcl=a7f27047b8146a699248316ca0d4495f7a9f9a99

This constraints tasks to a single app and a fixed list of arguments. In particular, it blocks an effort to run multiple simulators in parallel within a task, which may greatly improve our capacity utilization.

I propose to create an isolate file per task dynamically, without going through a common fixed template.