Mention blacklist when gpu compositing or raster is disabled.
Mention blacklist when gpu compositing or raster is disabled.

R=kbr@chromium.org
BUG=600499

Review URL: https://codereview.chromium.org/1899693004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389293}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk