Re-add RASTER_SCALE to prevent GPU memory exhaustion.
Re-add RASTER_SCALE to prevent GPU memory exhaustion.

This is a speculative fix for
https://code.google.com/p/trace-viewer/issues/detail?id=326.

I suspect removing the RASTER_SCALE parameter in
https://code.google.com/p/trace-viewer/source/detail?r=760 regressed
the story.json test file. Until we remove the canvas padding in
https://code.google.com/p/trace-viewer/issues/detail?id=228, this
scale is required to prevent GPU memory exhaustion.

BUG=326
TBR=nduca

Review URL: https://codereview.appspot.com/11621043
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