Ensure we don't load plugins on the IO thread.

I had to move the locks from PluginService to PluginList, so that a lock (which can block other threads) isn't held while loading the plugins.

BUG=17938
TEST=added asserts which crash if plugins loaded on IO thread, current UI tests exercise them
Review URL: http://codereview.chromium.org/164305

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@23501 0039d316-1c4b-4281-b951-d872f2087c98
26 files changed