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We are relying on lastchange.FetchVersionInfo in build_sdk.py. The revision
changed from being an SVN revision to a git sha, but we want to be able to
compare the values numerically.

This change modifies build_version.py to get "Cr-Commit-Position", which for
our purposes is functionally equivalent to the SVN revision.

BUG=none
R=bradnelson@google.com, bradnelson@chromium.org

Review URL: https://codereview.chromium.org/507883003

Cr-Commit-Position: refs/heads/master@{#292179}