Fix NullPointerException when connected but no video frame decoded

This fixes a crash seen when connecting to a host from the Android
Chromoting client. The painting code got called before a video frame had
been decoded, and tried to dereference a null Bitmap.

NOTRY=true

Review URL: https://codereview.chromium.org/47873012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233025 0039d316-1c4b-4281-b951-d872f2087c98