Introduce TextFinder class for decoupling WebFrameImpl and text finder.

It will be lazily initialized when user activates find on page functionality.

First part of this patch that adds TextIterator.{h,cpp} files is here:
https://codereview.chromium.org/181003010

BUG=348972

Review URL: https://codereview.chromium.org/181013007

git-svn-id: svn://svn.chromium.org/blink/trunk@170329 bbb929c8-8fbe-4397-9dbb-9b2b20218538