[Cronet] Add documentation on runing native tests.

As part of this, refactor the checkout and build from the Android docs into
a common doc referenced by both Android and native.

Bug: 786559
Change-Id: Ibc6ebb299d878cd27c2d88af2d6366bf5b0aa5ce
Reviewed-on: https://chromium-review.googlesource.com/c/1363439
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616384}
5 files changed