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Makes test runner run XCTest based unit tests with correct test app, by
testing args passed to test runner.

Use xctest arg to determine runner type only, but not parser type
anymore in test runner, except for special handling for WebRTC xctests
with a TODO.

Related CLs:
- GN building the arg to test runner: crrev/c/2314134.
- GN config change in some iOS FYI builders: crrev/c/2315638.

Bug: 1001667, 1006881
Change-Id: I3fcd69f21641894f12e315898ce29059449b1b6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315245
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792412}