Adds desktop notification support for renderer process, and enables the build flag for chromium build of webkit. Doesn't expose the feature to any websites yet.

      BUG=none
TEST=none (This is part 1 of 3 for this feature; tests will follow).

Review URL: http://codereview.chromium.org/194079


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@27973 0039d316-1c4b-4281-b951-d872f2087c98