GitLab

When the mime_type_handler key is available in an extension, this creates a
BrowserPlugin and gives the ID of it to the background page.

BUG=368514

Review URL: https://codereview.chromium.org/263513004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276841 0039d316-1c4b-4281-b951-d872f2087c98