Revert r160178 "Remove code in Editor::rangeOfString() that unnecessarily resets the searchRange after the first find operation."

It breaks FindInPageControllerTest.IncognitoFindNextSecret:
http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/31909/steps/browser_tests/logs/stdio

TBR=jsbell
BUG=

Review URL: https://codereview.chromium.org/34473003

git-svn-id: svn://svn.chromium.org/blink/trunk@160222 bbb929c8-8fbe-4397-9dbb-9b2b20218538