The function WebPluginListBuilderImpl::addMediaTypeToLastPlugin() in WebKit/chromium/src does not initialize the pluginIndex field of the MimeClassInfo struct.
Quick fix coming shortly.
(CC-ing last editor according to 'git blame')
Created attachment 57201 [details]
patch: initialize pluginIndex field, use .at() instead of  for indexing mimes
Created attachment 57206 [details]
patch - quick fix: fix crashing part only
Patch landed unreviewed to fix the crashing issue.