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When the user cancels a save operation, chrome.fileSystem.chooseEntry()
does not pass back a valid entry. Use chrome.runtime.lastError to check
for this condition.

Change-Id: I54e655ec74b4d3c07f06cf981677751a2d38acab
Reviewed-on: https://chromium-review.googlesource.com/c/1446867Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: dstockwell <dstockwell@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627693}