Clarify WebResourceRequest.isForMainFrame docs.

Make it more explicit that isForMainFrame() returns true only for the
actual main frame document, and not for any subresources, even if they
are subresources of the main frame.

Test: make ds-docs
Change-Id: I942cbe6a8f50d2f3af00b99b14a8503c8c4556de
1 file changed