[IFrame DLP Rules Support] Populate frame_url_chain for upload content analysis requests

Add the chain of nested iFrames for upload trigger. Other triggers will be added later.

This is entirely gated by a finch flag, will have subsequent CLs for unit tests/browsertests and UMA metrics.

Bug: 397480137
Change-Id: I269cbbabd2d73e8f839004136f35ad355b9bf742
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6575500
Commit-Queue: Haihan Chen <haihan@google.com>
Reviewed-by: thefrog <thefrog@chromium.org>
Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Cr-Commit-Position: refs/heads/main@{#1464094}