Capture snapshots of File metadata whenever we send a File to IDB.  This way if someone changes a File during a long, slow commit, we won't surprise the user by storing the new version.

Also includes missing error-checking in a test that's sensitive to changes in this area.

See https://codereview.chromium.org/333533002/ for the Chromium code that will check the metadata if you're curious.

BUG=none

Review URL: https://codereview.chromium.org/325383002

git-svn-id: svn://svn.chromium.org/blink/trunk@176538 bbb929c8-8fbe-4397-9dbb-9b2b20218538