clang/win/gn: Pass -msse flags to clang-cl.exe for skia
clang/win/gn: Pass -msse flags to clang-cl.exe for skia

This is a port of https://codereview.chromium.org/475273002 to gn.

Also add a comment that explains that the skia_opts target in the GN build
isn't correct.

BUG=491209,82385,496512

Review URL: https://codereview.chromium.org/1155243010

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#333091}

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