Oilpan: move TextFinder to the heap.

With WebLocalFrameImpl on the heap, this object no longer needs to use
persistent (or untraced) references.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/674773003

git-svn-id: svn://svn.chromium.org/blink/trunk@184332 bbb929c8-8fbe-4397-9dbb-9b2b20218538