cc :  Delete LayerImpl::opacity

Since property trees have the required opacity info.

To to this, this CL : (opacity = LayerImpl::opacity)
- Removes pushing opacity at commit and activation.
- Removes SetOpacity in OnOpacityAnimated
- Removes SetOpacity in ScrollbarImplBase::PushPropertiesTo
- For tests, adds opacity to LayerImplTestProperties.

BUG=609205
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2018833002
Cr-Commit-Position: refs/heads/master@{#397000}
20 files changed