cc: Remove the round-about Callback for creating a TestContextProvider.

I did this already in the production ContextProviders but missed our
TestContextProvider. Passing a Callback just to have the Callback
run synchronously and use the return value is silly. We can just
pass the return value directly.

R=jamesr
BUG=

Review URL: https://codereview.chromium.org/23464093

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225746 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225746