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This adds requireAsync, which returns a promise for the requested
module, allowing extension modules to asynchronously import AMD modules:

requireAsync('foo').then(function(foo) {
  // Use foo.
});

BUG=390397

Review URL: https://codereview.chromium.org/359413004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281957 0039d316-1c4b-4281-b951-d872f2087c98