[docs] Update documentation about sources assignment filter

All of Chromium and third-party dependencies have been converted
away from sources assignment filter, so update the documentation
to say that the conversion is complete.

Bug: 1018739
Change-Id: Ia47e6e6054744e5aeaccef638e5b081af1abdfe7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471418
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817047}

docs/no_sources_assignment_filter.md

@@ -4,52 +4,15 @@ There is a [strong][0] [consensus][1] that the set_sources_assignment_filter
 feature from GN is a mis-feature and should be removed. This requires that
 Chromium's BUILD.gn file stop using the feature.
 
-This conversion is now complete. There are a few straggler calls to
-`set_sources_assignment_filter([])` that are no longer needed, but
-the Chromium build now no longer sets a default sources assignment filter,
-and all build files must manage `sources` with explicit if statements.
-
-## Why convert
-
-When set_sources_assignment_filter is called, it configures a list of patterns
-that will be used to filter names every time a variable named "sources" is
-assigned a value.
-
-Historically, Chromium used to call this function in build/BUILDCONFIG.gn thus
-causing the patterns to be applied to every BUILD.gn file in the project. This
-had multiple drawbacks:
-
-1.  the configuration of the list of patterns is located far from the point
-    where they are applied and developer are usually confused when a file
-    they add to a rule is not build due to those pattern
-
-2.  the filtering is applied to every assignment to a variable named "sources"
-    after interpreting the string as a relative filename, thus build breaks if
-    one of the forbidden pattern is used in unexpected location (like naming
-    the build directory out/linux, or having mac/ in path to SDK, ...)
-
-3.  the filtering is applied to every assignment to a variable named "sources"
-    in the whole project, thus it has significant negative impact on the
-    performance of gn
-
 Since October 2020, the filter is no longer used.
 
-## Conversion pattern
-
-To convert a BUILD.gn file it is necessary to change the following:
+Chromium build does not set a default sources assignment filter, and all build
+files must manage `sources` with explicit `if` statements.
 
-```
-  source_set("foo") {
-    sources = [
-      "foo.h",
-      "foo_mac.mm",
-      "foo_win.cc",
-      "foo_linux.cc",
-    ]
-  }
-```
+## Explicit assignment
 
-to
+If you have a target that have platform specific implementation files, you can
+use the following pattern:
 
 ```
   source_set("foo") {
@@ -74,9 +37,5 @@ to
   }
 ```
 
-Since the second pattern never assign a name that will be filtered out, then
-it is compatible whether the set_sources_assignment_filter feature is used or
-not.
-
 [0]: https://groups.google.com/a/chromium.org/d/topic/chromium-dev/hyLuCU6g2V4/discussion
 [1]: https://groups.google.com/a/chromium.org/d/topic/gn-dev/oQcYStl_WkI/discussion