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This CL updates version number to 6.0.0. On a second thought, I think
jumping the number to major release level is more clear.

Bug: 935051
Test: None
Change-Id: I735c7214c395da1043ff188ff7072c9880e22b19
Reviewed-on: https://chromium-review.googlesource.com/c/1485622Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Sheng-hao Tsao <shenghao@google.com>
Cr-Commit-Position: refs/heads/master@{#635412}