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This is to prevent "directory traversal attack", e.g. when the
caller provides a path such as:
"../../../../../var/log/chrome/wp.jpg".

Bug: 908304
Test: Using the test extension provided in the bug.
Change-Id: I5fc699963cc32c85042d065d19e3842566cde5f1
Reviewed-on: https://chromium-review.googlesource.com/c/1351426Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610961}