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For consistency, has_next is changed to has_more. Also, has_more is easier to
understand in providing extensions context.

TEST=browser_tests, unit_test: *FileSystemProvider*
BUG=373165

Review URL: https://codereview.chromium.org/304533003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274217 0039d316-1c4b-4281-b951-d872f2087c98