Ability to override .cc files completely

[bbondy]

We have the ability to override .h files via the include search path but not the .cc files currently.
For any .cc file that exists in chromium_src subdir, we should use it instead of the src chrome verison of it.

• Override the paths if the path exists using a python script and with cc_wrapper.
• Remove the patches we have with .cc overrides and use this instead.