Add partitioned native library generation tooling

This change builds on the prototype posted by pcc@, here:
http://crrev.com/c/1521283

The partitioned_shared_library GN template may be used in place of
shared_library to create a set of libraries linked simultaneously.  This
will be leveraged in later changes to split code into Android dynamic
feature modules.  The current change adds the GN template for the set of
partitions, along with the Python wrapper script invoked by GN.

Bug: 921663
Change-Id: I805d598b03ae15bd4ceeb8e915526ed8b15af4cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1568772
Reviewed-by: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654117}
4 files changed