[oilpan]: Remove support for tracing off-heap hashmaps.

With this change we can no longer trace off-heap hashmaps. To make this work I changed the ExecutionContext to be WillBeHeapSupplementable and moved the IDBPendingTransactionMonitor to the heap. Finally I changed the way we trace through supplementables. Specifically we only trace our supplements for HeapSupplementable and PersistentHeapSupplementable.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org, sigbjornf@opera.com
BUG=

Review URL: https://codereview.chromium.org/403333002

git-svn-id: svn://svn.chromium.org/blink/trunk@178648 bbb929c8-8fbe-4397-9dbb-9b2b20218538