A plugin for integrating Chromium CodeSearch is at
https://github.com/chromium/vim-codesearch
The plugin has support for performing free text search, cross reference lookup, navigating call graphs, and more. Installation instruction, screencasts and code can be found at the above URL.
Here's what a search buffer looks like:

Please direct bug reports to the respective GitHub projects' Issues.
Here for the Vim plugin, and
here for the library.
Thanks to Josh Karlin who wrote a
plugin integrating CodeSearch into Sublime whose code was the basis for the library.