Oilpan: make FrameLoadRequest be a stack allocated object.

Simplify this stack allocated object's handling of heap references.

R=ager@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/328113004

git-svn-id: svn://svn.chromium.org/blink/trunk@175953 bbb929c8-8fbe-4397-9dbb-9b2b20218538