DevTools: fix raster task event name used in Timeline

We used to convert RasterTask events into Rasterize timeline events but there is no such conversion in tracing based Timeline.

BUG=361045

Review URL: https://codereview.chromium.org/488673002

git-svn-id: svn://svn.chromium.org/blink/trunk@180638 bbb929c8-8fbe-4397-9dbb-9b2b20218538