GitLab

This CL implements startCopy and cancelCopy of fileBrowserPrivate APIs.
Note that startCopy works now, but cancellation and progress reporting are not
supported yet.

BUG=279288
TEST=Ran unit_tests and tested manually.
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/23604042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222007 0039d316-1c4b-4281-b951-d872f2087c98