Data Sharing: Add unit test for DataSharingPageHandler

Bug: 342638650
Change-Id: I7e82b256d9f3c1c0e2c186abc581e5435878aea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5825452


Commit-Queue: Yuheng Huang <yuhengh@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: pengchao Cai <pengchaocai@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349489}