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This is a reland of b84a396f

Original change's description:
> Add InterestFeedV2 feature flag
>
> If enabled, the v2 feed stream loading task is kicked off when the NTP
> or start surface is created. Not currently user-visible; this is
> happening silently while the existing feed is created and shown.
>
> The binary size increase is mainly due to generated proto code that was
> added previously but has not been linked until now. We are generating
> the sources with "option optimize_for = LITE_RUNTIME;" which matches the
> existing feed library and provides an acceptable speed/size tradeoff.
>
> Once the v2 feed component is fully launched, we can begin removing the
> existing feed library. For a recent measurement of the binary size we
> would eliminate by removing the existing feed library, see this CL:
> https://chromium-review.googlesource.com/c/chromium/src/+/1829821.
>
> Binary-Size: The increase is temporary (see above).
> Bug: 1044139
> Change-Id: Ic596d28e03300ad617f98f0b725cdf5ed1b0b986
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132648
> Reviewed-by: Dan H <harringtond@chromium.org>
> Commit-Queue: Ian Wells <iwells@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#756004}

Binary-Size: Temporary. See the original CL description.
Bug: 1044139
Change-Id: I820a43891b7db4e0b335a51e51fe452fe325f44c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134936
Commit-Queue: Ian Wells <iwells@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756314}