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Added corresponding unit tests for new conversions.

This change is needed in order to support new changes
to ScrollTimeline, which needs to be able to support
-infinity and infinity values. The infinity values
were already covered, but -infinity values were not.

https://chromium-review.googlesource.com/c/chromium/src/+/2028173

Change-Id: I634860353782cf56a560228bf37903395b400dc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2038130Reviewed-by: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Jordan Taylor <jortaylo@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#742689}