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This is a reland of https://codereview.chromium.org/659533002/

Previous CL broke telemetry unittests and perf unittests because it
didn't handle properly shell commands which may end with a newline
character when check_rc is not None.

This revised patch strips any whitespace at the end of shell commands
to prevent this.

BUG=267773

Review URL: https://codereview.chromium.org/669573003

Cr-Commit-Position: refs/heads/master@{#300322}