Add brotli to .vpython3 for depot_tools

This is now a required package for requests. Add this also to
depot_tools and it fixed the issue in wpt importer.

Bug: 1453060
Change-Id: I5bece6f40d738c7e6af2720646965b573c5493e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4602125
Commit-Queue: Weizhong Xia <weizhong@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

.vpython3

@@ -47,6 +47,10 @@ wheel: <
   name: "infra/python/wheels/idna-py2_py3"
   version: "version:2.8"
 >
+wheel: <
+  name: "infra/python/wheels/brotli/${vpython_platform}"
+  version: "version:1.0.9"
+>
 
 # Used by:
 #   my_activity.py