Currently we can't merge tough_pinch_zoom_cases to rendering.mobile (issue 852373) because the stories in that story set use the 'desktop' user agent (to test rendering performance of desktop content on mobile), whereas the rest of rendering.mobile uses 'mobile' user agent.

Part of the reason why "user agent" is set at page_set/shared_state level is because in the past, we don't restart browser in between runs. So once you start the browser to run the test, you are stuck with a single user agent (user agent is specified as browser startup flag in https://cs.chromium.org/chromium/src/third_party/catapult/telemetry/telemetry/internal/backends/chrome/chrome_startup_args.py?rcl=985295aafdbfaecd1f27cb6357871f616ca92c99&l=60).

Today, we almost always restart browser between tests (https://cs.chromium.org/chromium/src/third_party/catapult/telemetry/telemetry/page/shared_page_state.py?rcl=985295aafdbfaecd1f27cb6357871f616ca92c99&l=167), so it should be able to possible to move user_agent to be property of page (https://cs.chromium.org/chromium/src/third_party/catapult/telemetry/telemetry/page/__init__.py?rcl=985295aafdbfaecd1f27cb6357871f616ca92c99&l=18)



Another way to approach this is also look into see if there is devtool proptocol that allows us to update user agent without restarting browser.