Set root resource ID upon full feed update.

Set root resource ID upon full feed update.

We used to set the resource ID at DriveResourceMetadata's
constructor as for WAPI the resource ID for root is
a fixed string, but for Drive API it is not fixed and
was set when About resource is obtained.

This caused a trouble for maintaining the code and
this CL is trying to consolidate setting root resource
ID at one place.

Although for Drive API 'root' can be used for a convenient
alias for queries, Drive API usually returns a reference
to root directory (e.g. reference to parent folder)
as its real root directory resource ID rather than 'root'
alias, therefore we don't use it here.

BUG=152230, 152288, 157314
TEST=compiles, pass all existing tests.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=164876

Review URL: https://chromiumcodereview.appspot.com/11227020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@165219 0039d316-1c4b-4281-b951-d872f2087c98