OpenFileSystem should accept not mount_type but lower filesystem type

FileSystemContext::ResolveURL which will be enabled by the Blink side change[1]
passes not a mount_type (ex. kFileSystemTypeExternal) but a lower filesystem
type (ex. kFileSystemTypeNativeLocal) into FileSystemBackend::OpenFileSystem.

[1] https://codereview.chromium.org/24376002/


BUG=177137,297412
TEST=browser_tests --gtest_filter=FileDisplay/FileManagerBrowserTest.* (with Blink patch)
TEST=unit_tests --gtest_filter=ChromeOSFileSystemBackendTest.*
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/24373002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224991 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224991