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This new functionality is hidden behind both a GN arg
(enable_run_ios_unittests_with_xctest) and a commandline switch
(--enable-run-unittests-with-xctest), in order to default it to off
until the bots are updated to properly run XCTest-based unittests.

The iOS test runner is updated to run in one of two modes.  When
--enable-run-unittests-with-xctest is false (the default), behavior is
unchanged; TestSuite::Run() calls UIApplicationMain(), then the
UIApplicationDelegate calls TestSuite::Run() again, which actually runs
the tests when invoked the second time.  When the switch is set to true,
the second invocation of TestSuite::Run() is made by our XCTestCase
subclass rather than by the application delegate.

Xcode provides the ability to run XCTests and XCUITests from the
commandline, but does not provide any other way to install and run an
app outside of this test-based workflow.  (We had an alternative that
used third party libraries, but they no longer work on iOS 13.)  This
makes it difficult to install and run GoogleTest-based tests on iOS
devices, since they run as a single self-contained application, but it
would not be practical to drop GoogleTest support on iOS.  Instead, we
are exploring invoking these tests via XCTest, which would allow us to
use Xcode's tooling but still run the same GoogleTest-based tests as on
other platforms.

BUG=635509

Change-Id: I26c67d9c7e16a744f43a20f2d8c5839ca8b3c31a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787593Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694749}