Cherry-pick: CC: Cap the size of the SOON_BIN.
Cherry-pick of Chromium crrev.com/r227711.
ALLOW_PREPAINT_ONLY is used to reduce memory usage, but
currently the SOON_BIN can still grow to an unbounded size
because we will schedule rasters for the entire SOON_BIN,
even though our paint rate is what truly determines if we
Minus raster tasks spikes seen in http://crbug.com/303397,
this keeps memory usage well below the limit when using
1 file changed