[Telemetry] Remove a 2.7 polyfill now that we depend on 2.7.

BUG=

Review URL: https://codereview.chromium.org/155483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249487 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249487