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This reverts commit 9384d0a2.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [ios] Remove Unused FullScreen Feature Flags
> 
> There is no need for ViewportAdjustmentExperiment since the only experiment
> remaining is just the kSmoothScrollingDefault feature flag.
> GetActiveViewportExperiment() can just be a function that returns
> whether or not smooth scrolling is turned on. GetActiveViewportExperiment()
> and ViewportAdjustmentExperiment are still needed for downstream compatibility,
> and will be removed in a subsequent CL.
> 
> Bug: 914042
> Change-Id: I57bcd13dafaafb3b3a05ec65f23dacf2936e81e9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799663
> Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
> Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#697673}

TBR=kkhorimoto@chromium.org,thegreenfrog@chromium.org

Change-Id: Id1e0df1c06297d0249717102d5dc87e6343b6b63
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 914042
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812091Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697840}