Allow someone interested in checking out a subset of the PDFium source
to do so via:

fetch pdfium --checkout_configuration=small

Along the way, also add support for the target_os and target_os_only
properties, which are carried over from the Chromium fetch config.

Change-Id: I11153fe7cb828c748055328ab2b60224b665ae84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3758798
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>