DevTools: Treat backslashes in file path correctly when searching across all files on windows.

R=yurys, pfeldman
BUG=275401

Review URL: https://chromiumcodereview.appspot.com/22832003

git-svn-id: svn://svn.chromium.org/blink/trunk@156471 bbb929c8-8fbe-4397-9dbb-9b2b20218538