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Chrome builds with -msse2 globally these days, so remove the skia_chrome_opts
target and fold it into skia_chrome.  Keep skia_opts but simplify the
comments there a bit.

No intended behavior change.

BUG=496512,348761

Review URL: https://codereview.chromium.org/1161853008

Cr-Commit-Position: refs/heads/master@{#333129}