Ensure that FileSelectHelper is destroyed on the UI thread.
Ensure that FileSelectHelper is destroyed on the UI thread.

R=sky
BUG=563932

Review URL: https://codereview.chromium.org/1616563002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370787}

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