Quick Answers: Add browser client

Add the browser client to get Access token.

Bug: b/150034512
Test: Manual Test
Change-Id: Ic3f87ee4299779797005d56daa411b122750bea7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429528


Commit-Queue: Yue Li <updowndota@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811079}