content: Limit number of raster threads to 4.
content: Limit number of raster threads to 4.

I have not been able to find a page or machine that benefits from
more than 4 raster threads. Using more than 4 raster threads seem
to instead reduce responsiveness as we have more raster work in
flight that can't be canceled. This reduces the number of raster
threads until we have a better mechanism in place to prevent
pre-paint work from slowing down non-raster work.

BUG=504515

Review URL: https://codereview.chromium.org/1211053003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336291}

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