[COPY] - Implemented API for tracking ongoing file transfers from file manager.

[COPY] - Implemented API for tracking ongoing file transfers from file manager.

new method:
chrome.fileManagerPrivate.getFileTransfers(function(transfers) {
});

new event:
chrome.fileManagerPrivate.onFileTransfersUpdated.addListener(function(transfers) {
});

where:

transfers = [
{
'fileUrl': 'filesystem://.../external/gdata/myfile.txt',
'transferState': 'started|in_progress|completed|failed',
'transferType': 'upload|download',
'processed': 1234,
'total': 54331
},
...
]

BUG=chromium-os:27819
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9703042

TBR=zelidrag@chromium.org
Review URL: https://chromiumcodereview.appspot.com/9719012

TBR=zelidrag@chromium.org