Add manifest_merger to DEPS.

It was introduced by [1] and it is required to roll Chromium into
WebRTC.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/3743741

Bug: None
Change-Id: Icf795a5500c07630ebb5a8b2b700caec964b3660
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/267700
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37425}
diff --git a/DEPS b/DEPS
index 4b02c55..7db9b52 100644
--- a/DEPS
+++ b/DEPS
@@ -380,6 +380,17 @@
     'dep_type': 'cipd',
   },
 
+  'src/third_party/android_build_tools/manifest_merger': {
+      'packages': [
+          {
+               'package': 'chromium/third_party/android_build_tools/manifest_merger',
+               'version': '0WkAedh1tJB8lzisWJRT80UjpacKLltuV7NqP-0tx9gC',
+          },
+      ],
+      'condition': 'checkout_android',
+      'dep_type': 'cipd',
+  },
+
   'src/third_party/android_sdk/public': {
       'packages': [
           {