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The directory names in chromium-browser-snapshots bucket have
changed recently, breaking all ChromeDriver builds. Changing
ChromeDriver waterfall script to compensate. In particular,
the following directory names have changed:

    Win -> win32_rel
    Mac -> mac_rel
    Linux_x64 -> linux_rel

Also removing obsolete code for 32-bit Linux.

TBR=crouleau@chromium.org

Change-Id: I2927152abc91a097fbb3c50693a578eaecc44752
Reviewed-on: https://chromium-review.googlesource.com/1157908Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579695}