Revert 31458 - Revert "Always destroy the ExtensionsService on UI thread."
Revert 31458 - Revert "Always destroy the ExtensionsService on UI thread."

It introduced memory leaks in tests. I have to ensure that
all message loops run pending tasks to guarantee that
ExtensionsService will be destroyed.

TBR=erikkay

Review URL: http://codereview.chromium.org/377034

TBR=phajdan.jr@chromium.org
Review URL: http://codereview.chromium.org/378029

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@31460 0039d316-1c4b-4281-b951-d872f2087c98

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