An unstated dependency between chromium and openjpeg:0 is created if that library is present while chromium is built. This is revealed when chromium is built as a binary package and distributed to a Gentoo system without openjpeg installed. On the bright side, I haven't experienced any issues with Chromium built against system-openjpeg. :-) Unfortunately I'm not familiar with the Chromium build system to a sufficient degree to suggest a fix...
I can't reproduce this. Could you make the bug report more precise by providing the following? - emerge --info - version number of chromium and USE flags - steps to reproduce the problem - how specifically does the problem manifest (verbatim error messages)
Feel free to reopen with requested info.