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Simplify the WebKit thread model.  It's now created/destroyed on the UI thread (before/after the IO thread is started/stopped).  The WebKit thread is created lazily as needed (while on the IO thread).TEST=noneBUG=none

Review URL: http://codereview.chromium.org/149238

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20109 0039d316-1c4b-4281-b951-d872f2087c98