GitLab

Second take.
Previous attempt (https://codereview.chromium.org/2440143002) caused
quality regressions. sSpatialLayers[0].sSliceArgument.uiSliceNum = 1 is
the workaround for now.

BUG=chromium:614970

Review-Url: https://codereview.chromium.org/2475623002
Cr-Commit-Position: refs/heads/master@{#430299}