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This cl splits the big CL with changes to net/ folder and
has changes only to net/cert_net according to the
changes to URLRequest API.

What the big cl does:
It modifies net/ clients that use URLRequest API as long as
URLRequest::Read returns int net errors and
URLRequest::Delegate and NetworkDelegate methods
(for example, OnResponseStarted or OnCompleted) have int
net_error in the arguments now.

The reason behind splitting the CL into small one is that
an android bot started to be unstable and unittests became
flaky. It was not possible to locate the problem and the
decision was to split the CL and upload small parts with a
6+ hours interval in order to make it possible to locate
the problematic code.

The big CL is located here -
https://codereview.chromium.org/2265873002/

BUG=423484

Committed: https://crrev.com/a74d3cbc572fa1c85cc2c0434a9f8e2f9cba0cd9
Review-Url: https://codereview.chromium.org/2330863002
Cr-Original-Commit-Position: refs/heads/master@{#418014}
Cr-Commit-Position: refs/heads/master@{#419394}