Reland "Disable the new tab-loading animation"

This reverts commit 742b8857c4dd0ab4550f7b2c1f200ce5be406b9c.

Reason for revert: Flaky tests should be fixed in r615470.

Bug: chromium:912543, chromium:913135, chromium:913784

Original change's description:
> Revert "Disable the new tab-loading animation"
> 
> This reverts commit 355b8185716292a8f145c77168ee89148ecabf31.
> 
> Reason for revert: Made several tests flaky: https://crbug.com/912543
> 
> Original change's description:
> > Disable the new tab-loading animation
> > 
> > Makes sure that a lot of animation-related code is bypassed when the
> > new-tab-animation flag is off. This should hopefully fix a couple of
> > performance regressions that have not yet been root caused so that they
> > don't go out with M72.
> > 
> > Bug: chromium:912328, chromium:905745, chromium:905918, chromium:910265
> > Change-Id: Id3f131db427eb3ee1618d6c9683fd5e47dc134e8
> > Reviewed-on: https://chromium-review.googlesource.com/c/1364212
> > Reviewed-by: Sidney San Martín <sdy@chromium.org>
> > Commit-Queue: Peter Boström <pbos@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#614199}
> 
> TBR=pbos@chromium.org,sdy@chromium.org
> 
> Change-Id: Ib4c022a255ad085c1716d3559a7f84dcb61c2785
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: chromium:912328, chromium:905745, chromium:905918, chromium:910265
> Reviewed-on: https://chromium-review.googlesource.com/c/1366359
> Reviewed-by: Jeffrey Yasskin <jyasskin@chromium.org>
> Commit-Queue: Jeffrey Yasskin <jyasskin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#614440}

TBR=jyasskin@chromium.org,pbos@chromium.org,sdy@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromium:912328, chromium:905745, chromium:905918, chromium:910265
Change-Id: I3981455a00f743fae535de4626179dcceab9b2c4
Reviewed-on: https://chromium-review.googlesource.com/c/1372236
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615581}
3 files changed