Switch Copy implementation from JS to HTML5 based C++ private API.
This CL switches the copy operation for Files.app from JS's one to C++'s.
Note that this just does switching. Unused code clean up will be done in
following CLs.
BUG=261492
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.
Review URL: https://codereview.chromium.org/
24987002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225915 0039d316-1c4b-4281-b951-d872f2087c98