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This patch adds support for closing files. It is currently a no-op, and it is
not yet bound to fileapi.

TEST=chromium: *FileSystemProvider*CloseFile*
BUG=248427

Review URL: https://codereview.chromium.org/279213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270116 0039d316-1c4b-4281-b951-d872f2087c98