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By default ChromeDriver creates new log file,
option '--append-log' allows to append log file.

Bug: chromedriver:586
Change-Id: Ibcbcad66a3afbf5bff0fbbf2046a1fc694395290
Reviewed-on: https://chromium-review.googlesource.com/1247027Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Tatiana Buldina <buldina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594523}