GitLab

This CL
* Adds new filesystem type kFileSystemTypeDeviceMediaAsFileStorage,
which is handled by chromeos::FileSystemBackend.
It uses DeviceMediaAsyncFileUtil, so the implementation is
shared with MTP support in mediaGalleries API (MediaFileSystemBackend).

We need different types since we need different handlings.
(Permission management for file handler extensions, unlimited CopyOrMoveValidator
for allowing non-media files, etc.)

* Adds monitoring by storage_monitor::RemovableStorageObserver in
file_manager::VolumeManager so that MTP device is detected and
mounted to the file manager.

BUG=363960

Review URL: https://codereview.chromium.org/246293011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266458 0039d316-1c4b-4281-b951-d872f2087c98