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This is API clean up. The mountType is being replaced by VolumeType. This CL
refactors private APIs about the flag.
Note that this is preparation to return Drive as a Volume.

BUG=279276
TEST=Ran unit_tests, browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.
R=benwells@chromium.org, kinaba@chromium.org, satorux@chromium.org, yoshiki@chromium.org

Review URL: https://codereview.chromium.org/23658009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221978 0039d316-1c4b-4281-b951-d872f2087c98