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This change creates a functional proxy resolver for Windows that relies
on WinHttp APIs. This is not in its final state, though. This CL
represents the simplest, least-work way to implement this proxy
resolution service. Work still to come includes:
- Complete NetLogs
- Better error reporting from WinHttp APIs
- Proxy Delegate
- Proxy retry info
- Assurance that the proxy configs we're receiving are from the system

The WindowsSystemProxyResolutionService is the object that external
callers use to resolve a proxy. These callers can keep track of the
status of this proxy resolution via a ProxyResolutionRequest,
implemented here as a WindowsSystemProxyResolutionRequest. The request
object is mainly intended to kick off a specific proxy resolution and
report a result via a caller-provided callback. Both of these objects
deal with a WindowsSystemProxyResolver.

The WindowsSystemProxyResolver is a new class that does all the required
interfacing with WinHttp APIs. It is a reference counted object created
by the WindowsSystemProxyResolutionService. Once created, it is used for
the lifetime of the WindowsSystemProxyResolutionService. Throughout that
time, the Resolver indirectly keeps a WinHttp session handle open. Each
WindowsSystemProxyResolutionRequest is given a reference to the new
resolver. When the Request object attempts to resolve a proxy for a
given URL, it'll call into the Resolver object. Under the hood, the
Resolver object creates a handle for the proxy resolution and runs the
async call to WinHttp. At any time, the Request object can choose to
ignore a pending result from the Resolver (ex: shutdown). The Resolver
object must be reference counted because async WinHttp calls call back
directly into the Resolver object, so it needs to stay alive as long as
we're waiting for an async response from WinHttp.

The only other new object is the WinHttpAPIWrapper, which is just a thin
wrapper over WinHttp APIs which we use to enable easier testing and
to simplify some interactions with WinHttp. A WinHttpAPIWrapper is owned
by a WindowsSystemProxyResolver. The Resolver object is the only object
that should interact with the Wrapper object in any meaningful way.
Internally, the WinHttpAPIWrapper keeps track of the opened WinHttp
session handle that we're using for the lifetime of the Resolver object.

This change includes tests for the WindowsSystemProxyResolver layer (by
mocking out the WinHttpAPIWrapper) and the
WindowsSystemProxyResolutionService (by mocking out the
WindowsSystemProxyResolver).

Bug: 1032820
Change-Id: Ic1c60033ff148e6e8f3708e37a2af366613fefac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159128
Commit-Queue: Nicolas Arciniega <niarci@microsoft.com>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790493}