linux: search Chrome-specific plugin directories before Mozilla
linux: search Chrome-specific plugin directories before Mozilla

As discussed in
http://groups.google.com/group/chromium-dev/browse_thread/thread/7a70e5fcbac786a9

Review URL: http://codereview.chromium.org/195044

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@25783 0039d316-1c4b-4281-b951-d872f2087c98

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