Revert "[prebuilt] Switch to standalone GN version"

This reverts commit 819527b3e4c83a039592d47dc10db116dc6f899f.

Reason for revert: Breaks incremental build for dart code.

Original change's description:
> [prebuilt] Switch to standalone GN version
>
> This should be functionally equivalent to the Chromium version.
>
> Change-Id: I21f8315eaa0aaa0a78e2a18d753873d6f9d24ba2

TBR=phosek@google.com,mcgrathr@google.com,scottmg@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: If29a667994207809cfa44ddb2b67e2b2a425b6eb
diff --git a/fuchsia.ensure b/fuchsia.ensure
index b803cd8..a8f8235 100644
--- a/fuchsia.ensure
+++ b/fuchsia.ensure
@@ -5,9 +5,6 @@
 $VerifiedPlatform linux-amd64
 $VerifiedPlatform mac-amd64
 
-# GN
-gn/gn/${platform} git_revision:d7111cb6877187d1f378bd231e14ffdd5fdd87ae
-
 # vpython
 infra/tools/luci/vpython/${platform} git_revision:b9c4670197dcefd8762d6e509302acd3efc6e303
 
@@ -20,6 +17,9 @@
 # Ninja
 infra/ninja/${platform} version:1.8.2
 
+# GN
+infra/tools/gn/${platform} gn_version:549249
+
 # Breakpad
 fuchsia/tools/breakpad/${os=linux}-${arch} git_revision:9eac2058b70615519b2c4d8c6bdbfca1bd079e39