Copy WebFrameImpl.{cpp,h} to TextFinder.{cpp,h}

This patch copies Source/web/WebFrameImpl.{cpp,h} into Source/web/TextFinder.{cpp,h}
for decoupling WebFrameImpl and text finder.

Another patch will complete decoupling.

BUG=348972

Review URL: https://codereview.chromium.org/181003010

git-svn-id: svn://svn.chromium.org/blink/trunk@168712 bbb929c8-8fbe-4397-9dbb-9b2b20218538