DevTools: make DOMNodeIds a trivial wrapper around WeakIdentifierMap

DevTools: make DOMNodeIds a trivial wrapper around WeakIdentifierMap

WeakIdentifierMap is intended for mapping between objects and their identifiers, and
there's currently no reason to generate ids externally, so:

- move id allocation/assignment to WeakIdentifierMap;
- implement Oilpan version of WeakIdentifierMap;
- move DOMNodeIds a trivial wrapper around WeakIdentifierMap;
- get rid of WeakNodeMap header, move the definition to DOMNodeIds.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193960

[caseq, 2015-04-16 17:54:50]

caseq@chromium.org changed reviewers:
+ pfeldman@chromium.org

[pfeldman, 2015-04-17 15:59:44]

lgtm

[caseq, 2015-04-17 16:01:07]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2015-04-17 16:02:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1089933004/1

[commit-bot: I haz the power, 2015-04-17 17:52:13]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=193960