Add a inspectUISourceCode method to the InspectorFrontendAPI

Add a method to inspect UI source code in order to open the devtools to a
particular resource, and focus on a given line and column.

For use in: https://codereview.chromium.org/23459008

BUG=21734

Review URL: https://chromiumcodereview.appspot.com/23816005

git-svn-id: svn://svn.chromium.org/blink/trunk@157902 bbb929c8-8fbe-4397-9dbb-9b2b20218538