Adding missing virtual keyword to PPB_VideoDecoder_Impl.
Adding missing virtual keyword to PPB_VideoDecoder_Impl.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6740028

git-svn-id: http://src.chromium.org/svn/trunk/src@80037 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 0e165e2f6bfb15a66e384f081c0313ec383c4e9d
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser