Do not accept negative values for brightness filter

Do not accept negative values for brightness filter and remove the
usecounter for it, which shows low usage (0.01-0.02%).

Behavior matches Firefox.

Intent to deprecate and remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/eIVnLFD_tPM

Bug: 776208

Change-Id: I55a49759f96cfd6fbddcca25e8bcbc9046444b26
Reviewed-on: https://chromium-review.googlesource.com/990772
Commit-Queue: Rob Buis <rob.buis@samsung.com>
Reviewed-by: Chris Nardi <cnardi@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552607}
16 files changed