Convert MemoryBenchmarkingExtension to gin::Wrappable

BUG=334679
R=dmikurube@chromium.org

Review URL: https://codereview.chromium.org/145033004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246570 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246570