Fix `gn help --root-pattern`

This help command didn´t work because the switch was not returned
by the switches::GetSwitches() command. This fixes that.

Change-Id: I26fd33924bebd4b0a4003c16d025ceeda8072d12
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/18720
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: David Turner <digit@google.com>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index e48e43d..f8c43a3 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -8392,6 +8392,7 @@
     *   --nocolor: Force non-colored output.
     *   -q: Quiet mode. Don't print output on success.
     *   --root: Explicitly specify source root.
+    *   --root-pattern: Add root pattern override.
     *   --root-target: Override the root target.
     *   --runtime-deps-list-file: Save runtime dependencies for targets in file.
     *   --script-executable: Set the executable used to execute scripts.

src/gn/switches.cc

diff --git a/src/gn/switches.cc b/src/gn/switches.cc
index 67c18a1..bf0c753 100644
--- a/src/gn/switches.cc
+++ b/src/gn/switches.cc
@@ -338,6 +338,7 @@
     INSERT_VARIABLE(NinjaExecutable)
     INSERT_VARIABLE(NoColor)
     INSERT_VARIABLE(Root)
+    INSERT_VARIABLE(RootPattern)
     INSERT_VARIABLE(RootTarget)
     INSERT_VARIABLE(Quiet)
     INSERT_VARIABLE(RuntimeDepsListFile)