When looking up a file in the first IDs list, and the file is outside of the tree starting at SRCDIR, just use the filename as key
When looking up a file in the first IDs list, and the file is outside of the tree starting at SRCDIR, just use the filename as key

BUG=none
TEST=building content_shell from within WebKit works for Release mode
Review URL: https://chromiumcodereview.appspot.com/10800072
camny125/grit-i18n,camny125/grit-i18n,imuse2012/grit-i18n,imuse2012/grit-i18n