Add 'kFeatureManagementHistoryEmbedding' flag.

This flag is meant to be enabled by the feature management module and
is used to gate the history embedding feature.

Bug: b:345243496
Change-Id: I2679f5ff6b200cab08d92b65cf179e6f00350a01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617576


Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313095}