Gracefully handle child process death in out-of-process plugin loading.

      This also queues requests to load plugins, based on http://codereview.chromium.org/8243010/.

BUG=100053
TEST=Install Sonix webcam driver on OS X Lion and try to load a Flash video. It plays.

Review URL: http://codereview.chromium.org/8318028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@106738 0039d316-1c4b-4281-b951-d872f2087c98