Revert "Use retargeted start point when starting composited transitions."

This reverts commit 6562517fa6e3a6d3a9690f36d3582e2fd23fa63a.

Reason for revert: I think this has an invalid cast and is causing CFI failures, see https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20CFI/9012 .

Original change's description:
> Use retargeted start point when starting composited transitions.
> 
> When starting transitions we check if they are interpolable, and don't
> start a TransitionInterpolation if not. When retargeting, the start
> keyframe from the previously active interpolation may cause an
> incompatible interpolation so we must take this into account.
> 
> Bug: 853698
> Change-Id: Iaeeba35c64c659818c68d30e8f999e704b382926
> Reviewed-on: https://chromium-review.googlesource.com/1126519
> Commit-Queue: Robert Flack <flackr@chromium.org>
> Reviewed-by: Majid Valipour <majidvp@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#573980}

TBR=flackr@chromium.org,majidvp@chromium.org

Change-Id: I4de74036835b09da2fe4751280bea9d9b0e205a0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 853698
Reviewed-on: https://chromium-review.googlesource.com/1132016
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574034}
3 files changed