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Revert "win: write a deterministic-ish timestamp into the PE/COFF header instead of the current time"

This reverts commit ef36dc19.

Reason for revert: This turns out to break the official android build, which apparently was relying on a broken aspect of the build that this fixed :(. See https://crbug.com/871173.

Original change's description:
> win: write a deterministic-ish timestamp into the PE/COFF header instead of the current time
>
> We used to set the timestamp to a hash of the binary, similar to
> https://blogs.msdn.microsoft.com/oldnewthing/20180103-00/?p=97705
> However, that caused an appcompat warning on Windows 7 to appear, which
> interpreted the hash as a timestamp. (It's possible that https://llvm.org/PR38429
> could help with that, but my guess it won't have an effect on Windows 7,
> which likely always believes that the the coff timestamp field always stores
> a timestamp).
>
> So currently we write the current time during linking in that field, but that's
> bad for build determinism and that in turn is bad for swarming test result cachability.
>
> build/write_build_date_header.py already creates a deterministic BUILD_DATE
> with several tradeoffs. Cachability wants this to change infrequently, but
> things like HSTS need a "real" build date and want this to change frequently.
> The compromise is: The date changes once per day in official builds, and
> once a month in regular builds.
>
> (We could use /Brepro in ldflags instead of /TIMESTAMP for unofficial builds to get
> the binary hash in the timestamp, but having the header timestamp match the BUILD_DATE
> define seems nice.)
>
> So let's use that same time as timestamp in the PE/COFF header. lld-link has a
> /TIMESTAMP: flag we can use to pass in an explicit timestamp.
>
> Since tools can't have deps, we need to compute the timestamp at gn time,
> so split write_build_date_header.py in two pieces: build/compute_build_timestamp.py
> that just prints the timestamp we want to use, and the old write_build_date_header.py, which
> now takes that timestamp and writes the header file.
>
> Call compute_build_timestamp.py at gn time so that we can pass it in ldflags, and
> pass the resultl to write_build_date_header.py which keeps running as an action
> during build time (so that we at least don't need to write a file at gn time).
>
> An additional wrinkle here is that the PE/COFF timestamp is used as one of just two
> keys per binary for uploading PE binaries to the symbol server, the other being file size.
> https://bugs.llvm.org/show_bug.cgi?id=35914#c0 has a good description of this, and
> tools/symsrc/img_fingerprint.py's GetImgFingerprint() is our implementation of it.
> But since we only upload binaries with symbols for official chrome builds to the symbol server,
> a timestamp that changes once a day should be still enough. (32-bit and 64-bit chromes
> have the same filename, and we might rarely build canary and beta and stable all on the
> same day, but them all being the same size seems highly unlikely.)
>
> Bug: 843199,804926,330260
> Change-Id: I1d4193cc537ae0c4b2d6ac9281fad29de754dd6c
> Reviewed-on: https://chromium-review.googlesource.com/1161104
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Hans Wennborg <hans@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#580585}

TBR=thakis@chromium.org,hans@chromium.org,dpranke@chromium.org
NOTRY=true

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 843199, 804926, 330260
Change-Id: Ib93697a82f8a9d3fb303b763609e82e0612887cd
Reviewed-on: https://chromium-review.googlesource.com/1166203
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581485}