Fix PathProvider to not modify result when DIR_TEST_DATA does not exist
Fix PathProvider to not modify result when DIR_TEST_DATA does not exist

Calling code expects result to be unmodified when PathProvider fails.

BUG=568225

Review URL: https://codereview.chromium.org/1513823002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#364740}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk