Oilpan: Make InspectorInstrumentationCookie stack-allocated

tiny-innerHTML.html allocates a ton of persistent handles in InspectorInstrumentationCookie().
I thought this might be a cause of the regression in tiny-innerHTML.html, but it wasn't.
Thus this is just a cleanup CL.

BUG=340522

Review URL: https://codereview.chromium.org/668453003

git-svn-id: svn://svn.chromium.org/blink/trunk@183940 bbb929c8-8fbe-4397-9dbb-9b2b20218538