Create a UIApplication when running tests on iOS.

Fix issue where running tests on iOS would get killed after a certain
time had passed. This seems to be due to springboard killing apps
that don't have a GUI running. Creating a UIApplication to wrap
the test suite seems to solve this problem in chromium.

This CL adds a class for this purpose. Most of the code was copied
from chromium with bits taken out.

Bug: webrtc:7161, webrtc:7758
Change-Id: I10f9bc8914e73f2870a9b0a2703cde496af8db2f
Reviewed-on: https://chromium-review.googlesource.com/528173
Reviewed-by: Henrik Andreasson <henrika@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Commit-Queue: Kári Tristan Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18509}