What is "isForMainFrame" request property meaning?

What is isForMainFrame meaning (android-21)? Is it the same as isMainFrame in android-19?

For "google.com" (which is immediately redirected to "google.ru" according to my locale) i'm getting the following log for android-19:

3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=true for http://www.google.com/
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dXfUdI
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dw0eut
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dXewaI
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dw0VUk
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dXfwM5
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dw0f1v
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dXehwz
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/1QKw9aI
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dXeyiN
3068-3088 D/IoThreadClientInvocationHandler: isMainFrame=false for http://ift.tt/2dw0DwJ

I expected to see true for all the resources since it's the resources from the main (and single) frame at "google.ru". Am i wrong?