LazyLoad: Mark automatic lazyloading as experimental

This CL marks automatic lazyload support as experimental. Restrict to
data saver feature is also marked experimental, since that is the
likely launch behavior.

Bug: 987870
Change-Id: Ia62276460de24b903222905a87b238f4feb31652
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720991
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681700}