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+ Turn on precompiled headers on Mac.
+ Change the toolchains to use asmflags instead of cflags and cflags_c.
+ Remove a TODO to hardcode the Mac SDK since the location of the SDK is not always guaranteed (on bots, for example).
+ GN: .S or .asm files no longer trigger cflags or cflags_c to be written to the ninja file.

BUG=none

Review URL: https://codereview.chromium.org/1368223002

Cr-Commit-Position: refs/heads/master@{#351202}