[css-typed-om] Add per-property test for 'opacity'.

This patch adds tests for opacity, which only takes a <number> and whose
computed value is clamped to [0, 1]. This is the <number> property so
we had to add to the test harness.

Bug: 545318
Change-Id: I5689843188811c8c14c062a60deb0a8c580de3d9
Reviewed-on: https://chromium-review.googlesource.com/927745
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: nainar <nainar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538269}
5 files changed