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

Revert of [oilpan]: Remove support for tracing off-heap hashmaps. (https://codereview.chromium.org/403333002/)

Reason for revert:
Seems to have broken compile across the tree.

Original issue's description:
> [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=
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178648

TBR=ager@chromium.org,erik.corry@gmail.com,haraken@chromium.org,oilpan-reviews@chromium.org,sigbjornf@opera.com,tkent@chromium.org,vegorov@chromium.org,zerny@chromium.org,wibling@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

[johnme, 2014-07-22 14:19:36]

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

[johnme, 2014-07-22 14:20:15]

The CQ bit was unchecked by johnme@chromium.org

[johnme, 2014-07-22 14:21:26]

It seems http://src.chromium.org/viewvc/blink?revision=178651&view=revision has
fixed this already.

[wibling-chromium, 2014-07-22 14:32:37]

On 2014/07/22 14:21:26, johnme wrote:
> It seems http://src.chromium.org/viewvc/blink?revision=178651&view=revision
has
> fixed this already.

Yes, the two changes (178647 and 178648) crossed each other where mine came in
last and had to be fixed. Sigbjorn was quick and kind enough to fix it in
https://codereview.chromium.org/413463002/ (revision 178651) :)