Remove support for specifying page_set attribute with file_path in benchmarks.
All benchmarks that uses the old format have been converted.
https://code.google.com/p/chromium/codesearch#search/&type=cs&sq=package:chromium&q=%22page_test%20=%20'%22%20
https://code.google.com/p/chromium/codesearch#search/&type=cs&sq=package:chromium&q=%22page_test%20=%20%5C%22%22
BUG=362293
Review URL: https://codereview.chromium.org/
331213003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278353 0039d316-1c4b-4281-b951-d872f2087c98