Adding src/third_party/androidx to the DEPS file.

Starting from [1], Robolectric requires androidx. This CL is needed
in order to unblock the Chromium roll into WebRTC [2].

[1] - https://chromium-review.googlesource.com/c/1436148
[2] - https://webrtc-review.googlesource.com/c/src/+/123284/

TBR=oprypin@webrtc.org

Bug: None
Change-Id: I6e36c242d72577c5eb27acba8d07d94b859607ae
Reviewed-on: https://webrtc-review.googlesource.com/c/123226
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26725}

DEPS

@@ -408,6 +408,17 @@ deps = {
       'dep_type': 'cipd',
   },
 
+  'src/third_party/androidx': {
+      'packages': [
+          {
+              'package': 'chromium/third_party/androidx',
+              'version': 'BgU0HKOH7unGo87kXkIKJlPMmaSOCFhvUKcIr9aborwC',
+          },
+      ],
+      'condition': 'checkout_android',
+      'dep_type': 'cipd',
+  },
+
   'src/third_party/sqlite4java': {
       'packages': [
           {