[GN]: Disable PCH on skia to unbreak GN Mac build.
[GN]: Disable PCH on skia to unbreak GN Mac build.

cmath and math.h don’t play well together when you want to call isfinite.
See the relevant GCC bug here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14608

Since precompile.h includes cmath and math.h, it can’t be enabled for skia in
its current state (and isn’t on the gyp build anyway)

BUG=540891

Review URL: https://codereview.chromium.org/1397763002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353118}

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