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This adds supports for building Cronet as a native shared library which can
be used through the C API generated from cronet.idl.

This is based on jamesr@'s work-in-progress to bring up the Cronet
library for Fuchsia:
https://chromium-review.googlesource.com/c/chromium/src/+/896389

Bug: 808075
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I598a8d1ca049e8c166e57546b7bdbf53e4659831
Reviewed-on: https://chromium-review.googlesource.com/914805
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Andrei Kapishnikov <kapishnikov@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537527}