git cl lint flags too aggressively for <string>.

Some source code has variable name as "string" which flags as it should be including <string>

Example:

third_party/WebKit/Source/core/inspector/InspectorOverlayAgent.cpp:959:  Add #include <string> for string  [build/include_what_you_use] [4]
Done processing third_party/WebKit/Source/core/inspector/InspectorOverlayAgent.cpp

There is no use of std::string on this file.

This is more critical on Blink where we should avoid using std::string. If <string> is added in this case it can lead people to believe that <string> can be used on this file, whereas it should be actually avoided.