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This CL adds support for copying files which are on the same provided file
system. Note, that copying between file system instances will be done using
FileStreamWriter.

TEST=unit_tests, browser_tests: *FileSystemProvider*Copy*
BUG=391362

Review URL: https://codereview.chromium.org/384303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284910 0039d316-1c4b-4281-b951-d872f2087c98