Fixed crash in WebFrameImpl::resetMatchCount function.
Now function ensureTextFinder is used to create TextFinder object if it doesn't exists.

BUG=359983

Review URL: https://codereview.chromium.org/229043004

git-svn-id: svn://svn.chromium.org/blink/trunk@175504 bbb929c8-8fbe-4397-9dbb-9b2b20218538