Restore implementation class for DOMStringMap.
Restore implementation class for DOMStringMap.

We still use it from custom code.

R=podivilov@chromium.org
BUG=3881
TEST=TODO

Review URL: https://chromiumcodereview.appspot.com//10704004

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@9167 260f80e4-7a28-3924-810f-c04153c831b5

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