Fix presubmit checks when --trybot-json flag is specified.
Fix presubmit checks when --trybot-json flag is specified.

Must change current working directory to the presubmit file's directory
to follow the contract of the presubmit scripts.

BUG=348691

Review URL: https://codereview.chromium.org/181063021

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@254618 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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