Update GN docs.

The README file is two years out of date. This removes the transition status
and references to GYP, and just retains the links to related documentation.

Fixes a help typo.

Change-Id: Ie783f32e90eb24643ba06698ddc0b3206116c724
Reviewed-on: https://chromium-review.googlesource.com/949769Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540994}

tools/gn/README.md

@@ -3,34 +3,6 @@
 GN is a meta-build system that generates [Ninja](https://ninja-build.org)
 build files so that you can build Chromium with Ninja.
 
-## Why did you switch from GYP?
-
-1. We believe GN files are more readable and more maintainable than GYP files.
-2. GN is fast:
-  * GN is 20x faster than GYP.
-  * GN supports automatically re-running itself as needed by Ninja
-    as part of the build. This eliminates the need to remember to
-    re-run GN when you change a build file.
-3. GN gives us better tools for enforcing dependencies (see
-   `gn check` and the `visibility`, `public_deps`, and `data_deps`
-   options for some examples).
-4. GN gives us tools for querying the build graph; you can ask
-   "what does X depend on" and "who depends on Y", for example.
-
-## What's the status of the GYP->GN migration for Chromium?
-
-_As of Oct 2016:_
-
-  * All of the Chromium builds have been switched over.
-  * Nearly all of the GYP files have been deleted from the Chromium repos.
-  * You can no longer build with GYP as a result.
-  * There are still some GYP files in place for the "Closure Compilation"
-    builders that need to be converted over.
-  * Some related projects (e.g., V8, Skia) may still support GYP for their
-    own reasons.
-  * We're still cleaning up some odds and ends like making gclient not
-    still use GYP_DEFINES.
-
 ## I want more info on GN!
 
 Read these links:

tools/gn/function_forward_variables_from.cc

@@ -105,8 +105,7 @@ const char kForwardVariablesFrom_Help[] =
   important because most targets have an implicit configs list, which means it
   wouldn't work at all if it didn't clobber).
 
-  The sources assignment filter (see "gn help "
-     "set_sources_assignment_filter")
+  The sources assignment filter (see "gn help set_sources_assignment_filter")
   is never applied by this function. It's assumed than any desired filtering
   was already done when sources was set on the from_scope.