Refactor private APIs for Volume related code.

As we're finishing refactoring around VolumeManager, we can simplify private
APIs, too.
- Removes unused fields from VolumeMetadata.
- getMountPoints is renamed to getVolumeMetadataList.
- Removes getVolumeMetadata.
- onMountCompleted now returns VolumeMetadata.

BUG=284975
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.

Review URL: https://codereview.chromium.org/24984002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226120 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226120