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This is sort of a reland of https://crrev.com/c/2260872, but rather than
calling bytes.decode() on every line we pass |universal_newlines=True| to
subprocess.Popen() so that |lines| is still a string and not a bytes object
in Python 3.

This approach does not cause the significant slowdown during linking that
led to the previous CL being reverted.

Bug: 941669
Change-Id: I92bc14d4dea01085f9f81153f610663960cc2fcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2263712
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781825}