Ninja has a deps cache where it reads .d files right after the command producing it has completed and stores the depfile contents in .ninja_deps in a fairly efficient binary format in the build dir. That way, ninja doesn't have to parse .d files from the disk at startup.

The deps cache is opt in. gn doesn't use it.

For a build of chrome on linux, only 182 depfiles get generated, and `-d stats` says parsing them only takes 2ms, so that's not a huge thing.

But on Android, out/gnand/toolchain.ninja contains 6502 depfiles, it might help more there. (But assuming the 2ms scale up linearly, still just 0.1s or so, so still not a huge win. I haven't tried `-d stats` on an android build.)

Might be nice to do for cleanliness still.