The first problem is figuring out how to pass --site-per-process cmdline flag from 1) build/android/test_runner.py into 2) bowels of //content (i.e. so that is is seen by content/common/site_isolation_policy.cc).  FWIW, it seems that test_runner.py's --test-arguments is gtest-specific - there is no equivalent for instrumentation tests.

For instrumentation tests, you're looking for --device-flags-file. The supplied file is currently expected to contain any command-line flags you want to pass w/ one flag per line.

Thank you for the pointer.  Are there existing uses of --device-flags-file that I could copy from?  For example - is there a preferred location and/or naming convention for those files?

What are your thoughts on adding support for --test-arguments for instrumentation tests (which could create a temporary device flags file behind the scene)?

Note that you can now pass arguments directly on the command line. Anything the test runner doesn't recognize will get passed on to the device.