iOS: cd into Documents folder at startup

CQ_INCLUDE_TRYBOTS=skia.primary:Test-iOS-Clang-iPadMini4-GPU-GX6450-arm-Release

Change-Id: I7beadad742bc9444491c7a315a827297a636d70d
Reviewed-on: https://skia-review.googlesource.com/8049
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
5 files changed