Adds ImageWriterPrivate.destroyPartitions operation.

BUG=328246

Review URL: https://codereview.chromium.org/109793006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242452 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242452