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This will replace //base/third_party/dmg_fp. dmg_fp takes at least one
mutex for every conversion, which is sub-optimal for performance. The
new double-conversion library comes from V8, and it is also used in
Blink. A future CL will move Blink onto this copy in //base.

Compatibility risk is high because things may rely on specific output
output behaviors of the old implementation.

Bug: 95729
Change-Id: I6b08093ff4e38e257629f1fcf1d4f095e170c24a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698345
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691618}