This is because of a documented, gpu compositing bug: Issue 164555: Flickering rectangles like a swarm of bees when watching youtube and vimeo videos. On affected systems, yellow snow is seen when viewing Flash videos and orange snow is seen when viewing HTML5 content.
It involves the Pepper version of Flash and certain Mesa drivers.
Running Chrome with the Pepper Flash but with the
--disable-accelerated-compositing switch stops the snow from appearing but WebGL function will be lost.
The other Flash, NPAPI Flash, also used by other browsers such as Firefox, isn't affected possibly because "the NPAPI plugin doesn't use the GPU process and probably just provides software rendered frames".
There are thus two workarounds:
disable the Pepper Flash and enable NPAPI Flash (if it is installed)
continue using the Pepper Flash but with the
Read the bug for full details.