Revert 248147 "Use volumeId instead of mountPath in fileBrowserP..."

> Use volumeId instead of mountPath in fileBrowserPrivate.removeMount()
> 
> Since mountPath is being deprecated, this patch makes the removeMount() use volumeId. This significantly simplifies the code.
> 
> TEST=Tested manually with an archive.
> BUG=316077
> 
> Review URL: https://codereview.chromium.org/135183013

TBR=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/151483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248189 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248189