Revert "flags: remove page-visibility-page-content-annotations"

This reverts commit 385a71875e0a9fd510c9a7f35a139c035e8dddf6.

Reason for revert: Failing PageContentAnnotationsServiceTest

Original change's description:
> flags: remove page-visibility-page-content-annotations
>
> This removes the flag and retains the default behavior.
>
> Fixed: 361124410
> Change-Id: I2c6347303afadc839ace7cfdb411641f63e7035c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7234456
> Reviewed-by: Zekun Jiang <zekunjiang@google.com>
> Auto-Submit: Mike Wittman <wittman@chromium.org>
> Commit-Queue: Zekun Jiang <zekunjiang@google.com>
> Cr-Commit-Position: refs/heads/main@{#1557732}

Bug: 361124410, 468402904
Change-Id: I611e9ba9db67d92f2fc370a9467ced8e85bda934
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7255406
Owners-Override: Joel Tan-Aristy <jtanaristy@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Joel Tan-Aristy <jtanaristy@google.com>
Cr-Commit-Position: refs/heads/main@{#1558334}