Move ElideString() from base/string_util.cc to app/text_elider.cc to

      reduce size of widely-included base libraries. 

Committing for tsepez.

BUG=49747
TEST=TextEliderTest.* 

Review URL: http://codereview.chromium.org/6017001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69555 0039d316-1c4b-4281-b951-d872f2087c98