Gallery: fix a bug that overwrite original checkbox does not work.
Gallery: fix a bug that overwrite original checkbox does not work.

BUG=521502
TEST=manually tested as described in the issue.

Review URL: https://codereview.chromium.org/1296013003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343647}

Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium