Moving gRPC support interfaces out of cronet and into a new component.

components/grpc_support/README.md

Index: components/grpc_support/README.md
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+gRPC Support
+===
+
+This directory contains the interface and implementation of the API to use
+an external network stack from gRPC. Chrome is the only current implementor
+of this interface, and so gRPC refers to this API as [Cronet] [https://github.com/grpc/grpc/blob/a8663fa55433732ea0f6e63c2fd586db0ea6f73f/src/core/ext/transport/cronet/client/secure/cronet_channel_create.c].
+
+Internally to Chrome, there will be two different backings of this API. Cronet

[blundell, 2016/10/05 11:09:03]

I'm confused: you say that this directory contains the implementation, and then
that there are two implementations, the first of which lives somewhere else. Is
the second the one in //components/grpc_support itself? That seems a little
confusing.

Higher-level: Why is there a need for two implementations? Naively, it seems
like Cronet-for-iOS-apps and Chromium could use the same impl, similar to how
Cronet consumes //net.

+(//components/cronet/ios), which is a library that allows iOS apps access to
+the Chromium networking stack, and Chromium proper. Implementation of the later
+is currently in progress.