Revert "Changed committed-interstitials flag from switch to feature."

This reverts commit 49040ae795592d3b0c6a5b03533565caeb9ae266.

Reason for revert: 

Findit (https://goo.gl/kROfz5) identified this CL at revision 574621 as the culprit
for introducing flakiness in the tests as shown on:
https://findit-for-me.appspot.com/waterfall/flake/flake-culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vNDkwNDBhZTc5NTU5MmQzYjBjNmE1YjAzNTMzNTY1Y2FlYjlhZTI2Ngw

Original change's description:
> Changed committed-interstitials flag from switch to feature.
> 
> Removed 'committed-interstitials' switch and added SSLCommittedInterstitials
> base::Feature. Name was changed to SSLCommmittedInterstitials since we are now
> doing separate rampups for each kind of committed interstitial. Switch was
> changed to Feature so that it can be enabled via variations.
> 
> Bug: 755632
> Change-Id: I0f3e0e01a94be2b515aa596827cd447d14c24b56
> Reviewed-on: https://chromium-review.googlesource.com/1118975
> Commit-Queue: Carlos IL <[email protected]>
> Reviewed-by: Jialiu Lin <[email protected]>
> Reviewed-by: Maksim Ivanov <[email protected]>
> Reviewed-by: Emily Stark <[email protected]>
> Reviewed-by: Giovanni Ortuño Urquidi <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#574621}

[email protected],[email protected],[email protected],[email protected],[email protected]

Change-Id: If9f9b15c8e3654dbd011104dfa469943bcd66e0c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 755632, 863272
Reviewed-on: https://chromium-review.googlesource.com/1136351
Reviewed-by: Mike West <[email protected]>
Commit-Queue: Mike West <[email protected]>
Cr-Commit-Position: refs/heads/master@{#574870}
21 files changed