Add Reason to ContentAnalysisRequest

GitHub equivalent for SDK:
https://github.com/chromium/content_analysis_sdk/pull/99

Bug: b/232967266, b/285317584
Change-Id: Iec80a9f04b8cde4518eabac03695e30b47d56c71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4789782


Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184829}