Oilpan: extend tracing over WebFrame trees.

Have the tracing also encompass the parent and opener.

Also refreshed some unrelated LocalFrame handling comments.

TBR=tkent
R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/594483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183241 bbb929c8-8fbe-4397-9dbb-9b2b20218538