[SyncFileSystem] Stop referring to DriveFileSyncService from extension/

[SyncFileSystem] Stop referring to DriveFileSyncService from extension/.

Adding SyncFileSystem::GetDefaultServiceName to break dependency from extension/ to DriveFileSyncService.

BUG=244775

[tzik, 2013-05-29 12:20:48]

DriveFileSyncService is originally a pluggable subsystem of SyncFileSystem.
So, I'd keep extension APIs independent from DriveFileSyncService.

yoz: PTAL to developer_private_api.cc.
kinuko, calvin: PTAL to others.

[kinuko, 2013-05-29 14:12:20]

https://codereview.chromium.org/16190005/diff/1/chrome/browser/extensions/api...
File chrome/browser/extensions/api/sync_file_system/sync_file_system_api.cc
(right):

https://codereview.chromium.org/16190005/diff/1/chrome/browser/extensions/api...
chrome/browser/extensions/api/sync_file_system/sync_file_system_api.cc:58:
sync_file_system::SyncFileSystemService::GetDefaultServiceName());
I think this parameter can be dropped. (Not used by the observer either)

https://codereview.chromium.org/16190005/diff/1/chrome/browser/extensions/api...
chrome/browser/extensions/api/sync_file_system/sync_file_system_api.cc:125:
service_name,
Ditto, I think this parameter can be dropped.

https://codereview.chromium.org/16190005/diff/1/chrome/browser/sync_file_syst...
File chrome/browser/sync_file_system/sync_file_system_service.cc (right):

https://codereview.chromium.org/16190005/diff/1/chrome/browser/sync_file_syst...
chrome/browser/sync_file_system/sync_file_system_service.cc:195: return
DriveFileSyncService::kServiceName;
Current service name (syncfs) is not even drive-specific, I think we can move
the kService name out of DriveFileSyncService.

Also given it's only used as a mount point name today we can probably rename
this method from GetDefaultServiceName() -> GetMountName().

[Yoyo Zhou, 2013-05-29 18:00:26]

developer_private_api LGTM

[calvinlo, 2013-05-30 05:04:55]

lgtm after Kinuko comments addressed