Currently, bisect-builds.py -a chromeos will place the local checkout's
icudtl.dat into the directory of the current revision being considered.
However, modern chromeos builds do contain an icudtl.dat file, and
overwriting the existing icudtl.dat file with a very recent icudtl.dat
file will cause the chromeos executable to segfault on startup.

This change simply checks to see if the icudtl.dat file already exists
and, if it does, does not try to overwrite it. Older bisects will
require running inside an older checkout.

Change-Id: I6927efc273e938c55f4067cd1708e7c58809545d
Reviewed-on: https://chromium-review.googlesource.com/598997


Reviewed-by: Prasad Vuppalapu <prasadv@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Wagner <bungeman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494414}