migrate GN docs from the wiki to the repo.

tools/gn/docs/faq.md

+# GN Frequently Asked Questions
+
+[TOC]
+
+## How will the build be converted?
+
+We intend to build a second independent build in parallel to the GYP
+build. Previous efforts to generate GYP as an intermediate stage proved
+difficult. There will be some smaller set of bots compiling this build,
+and we'll keep the GN build running on these configurations.
+
+## What is unsupported in GN?
+
+The main features not supported in GN yet are:
+  * Mac bundles
+  * Loadable module (this only matters on Mac where shared library !=
+    lodable module)
+  * Precompiled headers
+
+## Where is the GN documentation?
+
+Rather than on a separate wiki, it is versioned with the tool. Run `gn
+help`. See also the [quick start](quick_start.md) guide and the
+[language and operation details](language.md).
+
+## What is likely to break?
+
+Since common.gypi is not used for GN-generated GYP files, any rules
+there will no longer apply. There is a _lot_ of logic in there for many
+build configurations and various conditions where certain flags should
+or should not be used. Some of these build configurations aren't even
+supported any more. Some are run on other waterfalls or are used by
+individuals manually setting GYP\_DEFINES on their local system.
+
+## Will XCode/Visual Studio still be supported?
+
+They're not supported now.
+
+Long-term, if your use-case is to use Ninja for building but Visual
+Studio or XCode for debugging, there is desire to write a simple wrapper
+around Ninja that lists the files in Visual Studio or XCode format, and
+has a command to run ninja for when you press build. This setup should
+provide the type of interactive debugging experience people want (the
+iOS team currently uses a Ninja/XCode build like this with success).
+
+This project is not staffed. If you're interested, it probably isn't too
+hard. It won't get done unless somebody volunteers. There is a [spec for
+IDE
+integration](https://docs.google.com/document/d/1kwREU99u8GpRammLbbKwrfaDI6WV7nsMAaoF5dcuhOU/edit?usp=sharing).
+
+## I'm weird. Will my uncommon build mode be supported?
+
+One of the main benefits of the build changeover is that it will
+encourage us to refactor the build system. The project has generally not
+been as strict with build complexity and maintenance as we have with
+source code, and a lot of cruft has accumulated.
+
+In most cases, we will want to rethink how build flags are supported. We
+want to be more modular rather than throwing everything in the
+common.gypi equivalent. The bar for additions at this level will be very
+high, and we will need to figure out how to design certain build
+features. If you are interested in some weird configurations, this will
+likely make your life more difficult in the short term, but will
+hopefully bring long-term benefits for everybody.
+
+In some cases, we may decide that the overhead of supporting your build
+for BeOS running on a DEC Alpha is not in the interests of the project.
+There will likely be discussions about where to draw the line, and how
+to allow those who want to do weird things to do them cleanly without
+negatively affecting the rest of the Chromium community.
+
+## I'm only a little bit weird, will my development build flag be supported?
+
+Only if you do it yourself!
+
+Some features have build flags that turn on a debugging mode or switch
+between internal/external builds. This can be supported, but  as with
+GYP, your team will have to add an maintain the support.
+
+## I use supplement.gypi, what's the GN equivalent?
+
+Some projects use files called `supplement.gypi` to set build flags. GYP
+looks in each directory under `src` and adds merges these files into the
+build. The model is that then adding something to your gclient to add
+something to your build (e.g `src-internal`) automatically sets flags.
+
+This behavior is fragile and mysterious. Some people get build flags and
+they don't know why. If you remove the entry from your `.gclient` and
+don't remove the directory you'll be stuck on an old version of the
+flags/code and not know why. You can't have builds in the same checkout
+with the corresponding flags on and off. Some people and projects were
+abusing this behavior.
+
+In GN, such things should be done with build arguments (`gn args`) and
+configured on your build directory when you set it up. For some people,
+this will be an extra step. But it is explicit and clear, and you can
+have multiple builds in the same checkout with different flags.
+
+## How do I generate common build variants?
+
+In GN, args go with a build directory rather than being global in the
+environment. To edit the args for your `out/Default` build directory:
+
+```
+gn args out/Default
+```
+
+You can set variables in that file:
+
+  * The default is a debug build. To do a release build add
+    `is_debug = false`
+  * The default is a static build. To do a component build add
+    `is_component_build = true`
+  * The default is a developer build. To do an official build, set
+    `is_official_build = true`
+  * The default is Chromium branding. To do Chrome branding, set
+    `is_chrome_branded = true`
+
+## How do I do cross-compiles?
+
+GN has robust support for doing cross compiles and building things for
+multiple architectures in a single build.
+
+See [GNCrossCompiles](GNCrossCompiles.md) for more info.