InFile.load_from_files should try to load only '.in' files

InFile.load_from_files() should assert the input file is a '.in' file.

Review URL: https://codereview.chromium.org/397513003

git-svn-id: svn://svn.chromium.org/blink/trunk@178207 bbb929c8-8fbe-4397-9dbb-9b2b20218538