Fix a crash bug in ArcSelectFilesHandler

Don't let chrome crash when GetFileSelectorElements/OnFileSelectorEvent
gets called after SelectFileDialog is already closed
BUG=b:128261768
TEST=Manualy check that chrome doesn't crash

Change-Id: I32b98c280cfe0e472d4df28d08724ffc1e9129c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527943
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641823}
1 file changed