[net] Add documentation for feature process & supported platforms

net/docs/supported-projects.md

Index: net/docs/supported-projects.md
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+# Supported Projects
+
+The `//net` stack is used on a variety of platforms and in a variety of open
+source projects. These different platforms and projects have differing
+degrees of support from `//net` OWNERS as well as differing requirements for
+designs and technical requirements.
+
+Note that this is a rough high-level overview of the major projects; as more
+of `//net` is broken into consumable services/components as part of the Mojo
+servicificaiton efforts, it's likely that there will be a larger number of
+'variants' of these projects with different constraints and features.
+
+## Google Chrome Browser
+
+The Google Chrome browser, which lives in `//chrome`, is the most important
+`//net` consumer and shapes many of the core design decisions. In general,
+features that are not intended with or not compatible with the needs of
+the Google Chrome browser will have a very high bar for acceptance in `//net`.
+
+The feature matrix 
+
+  * **Supported Platforms**: Windows, macOS, Linux, Chromium OS, iOS, Android
+  * **Release Frequency**: ~6 weeks between releases
+  * **Automatic Updates**: Yes
+  * **Command-line Flags**:
+    * __Yes__: Windows, macOS, Linux, Chromium OS (Dev image), Android (rooted)
+    * __No__: Chromium OS (Release image), iOS, Android (Release)
+  * **Field Trials (Finch)**: Yes
+  * **Enterprise Policy**: Yes
+  * **User Metrics (UMA)**: Yes
+  * **Component Updater**: Yes
+
+## Chromium Browser
+
+The Chromium browser refers to the practice of certain Linux distributions to
+bundle the open-source components of Chrome Browser in `//chrome`, branded
+as Chromium. This version is not distributed by Google, but by individual
+Linux distributions (primarily). Other distributions based on building Chromium
+for other platforms exist, although do not see as wide of usage.
+
+  * **Supported Platforms**: Windows, macOS, Linux, Chromium OS, iOS, Android
+  * **Release Frequency**: Varies by distributor; some Linux distributions
+    treat versions of Chromium as "Long Term Stable" and support a single
+    version for a longer time than the Chromium projects do, others track
+    the Google Chrome release frequency.
+  * **Automatic Updates**: Varies by distributor
+  * **Command-line Flags**:
+    * __Yes__: Windows, macOS, Linux, Chromium OS (dev image), Android (rooted)
+    * __No__: Chromium OS (Release image), iOS, Android (Release)
+  * **Field Trials (Finch)**: No
+  * **Enterprise Policy**: Yes
+  * **User Metrics (UMA)**: Varies by distributor
+  * **Component Updater**: Varies by distributor
+
+## Android WebView
+
+Distinct from the Chromium browser, the Android WebView is itself based on
+the Chromium browser. On official Android devices running Android N or later,
+WebView is automatically updated when Google Chrome is updated on the
+device. For earlier devices, Android WebView is updated by the System WebView
+component.
+
+Android WebView may also be used on non-official Android devices, such as
+those based on the Android Open Source Project but do not go through the
+Android [Compatability Test Suite](https://source.android.com/compatibility/cts/).
+Such releases have limited to no interaction with the Chromium projects, and
+so their capabilities cannot be conclusively documented.
+
+For official Android devices, WebView has the following capabilities.
+
+  * **Supported Platforms**: Android
+  * **Release Frequency**: ~6 weeks between releases
+  * **Automatic Updates**: Varies. Updates are made available on the Android
+    App Store, but users must explicitly choose to update. As such, the
+    rate of update varies much more than for the Chromium browser.
+  * **Command-line Flags**: No
+  * **Field Trials (Finch)**: No
+  * **Enterprise Policy**: No
+  * **User Metrics (UMA)**: No
+  * **Component Updater**: No
+
+## `//content` Embedders
+
+In addition to Chromium, there are a number of other of embedders of
+`//content`, such as projects like [Chromium Embedded Framework](https://bitbucket.org/chromiumembedded/cef)
+or [Electron](http://electron.atom.io/). While `//net` does not directly
+support these consumers, it does support the `//content` embedding API that
+these projects use. Note that this excludes the
+[content_shell](../../content/shell) test framework.
+
+  * **Supported Platforms**: Windows, macOS, Linux, Chromium OS, iOS, Android
+  * **Release Frequency**: Varies by consumer; Officially ~6 weeks
+  * **Command-line Flags**: Varies by consumer
+  * **Field Trials (Finch)**: No
+  * **Enterprise Policy**: No
+  * **User Metrics (UMA)**: No
+  * **Component Updater**: No
+
+## Cronet

[Ryan Sleevi, 2017/02/16 00:58:30]

+mef: Could you review this for its capabilities and expand if necessary?

[mef, 2017/02/16 18:00:00]

Could you elaborate on 'capabilities'?

[Ryan Sleevi, 2017/02/16 18:03:55]

The points below and making sure I got them right

[mef, 2017/02/16 18:11:13]

Acknowledged.

+
+[Cronet](../../components/cronet/README.md) is a version of the `//net`
+network stack for use in mobile applications on iOS and Android. While
+primarily targetting first-party Google applications, Cronet's status as an
+open-source project, similar to the Chromium browser, means that it may
+find itself embedded in a variety of other projects.
+
+Unlike some of the other `//net` consumers, Cronet does not necessarily
+implement "The Web Platform" (as the holistic set of user agent-focused
+features), and instead is more akin to an HTTP(s) client networking library.
+
+  * **Supported Platforms**: iOS, Android
+  * **Release Frequency**: Varies

[mef, 2017/02/16 18:00:00]

Builds follow Chrome release schedule with daily Canary, Dev, Beta and Stable
channel builds matching Chrome for Android and iOS.
There are no official release availability for general consumers.  

[Ryan Sleevi, 2017/02/16 18:03:55]

Well, my understanding is that a build being available doesn't mean it's
integrated into a consuming product for potentially quite some time, is that a
fair statement? If so, the "When does this thing X that I landed here end up
affecting products" is ... an "it depends"?

[mef, 2017/02/16 18:11:13]

Correct. We generally make Dev channel builds available to internal consumers on
a weekly basis, but after that individual products follow their own release
schedule which 'Varies'.

+  * **Command-line Flags**: No
+  * **Field Trials (Finch)**: No

[mef, 2017/02/16 18:00:00]

There are 'experimental options' that consumers can configure based on their own
experiment framework.

[Ryan Sleevi, 2017/02/16 18:03:55]

Do those options use base::FieldTrial? My understanding is that no, it was a
series of explicit bools at the top-level interface.

[mef, 2017/02/16 18:11:13]

That's correct, those don't use base::FieldTrial.

+  * **Enterprise Policy**: No
+  * **User Metrics (UMA)**:
+    * __Yes__: First-party (Google) projects

[mef, 2017/02/16 18:00:00]

Proto-serialized UMA data is exposed via getGlobalMetricsDeltas() and
First-party apps have established pipeline to feed that into UMA server. This
doesn't preclude other consumers from pulling same data for separate processing.

[Ryan Sleevi, 2017/02/16 18:03:55]

No, but it does preclude other consumers from sending those to Google's UMA
dashboard, correct?

[mef, 2017/02/16 18:11:13]

I don't know, I was under impression that UMA is limited to first party.
To be clear, first party apps don't use the Chrome UMA mechanism to send data to
Google neither.

+    * __No__: General consumers
+  * **Component Updater**: No