WebGL: Never have more than one mailbox in the released state.

r165966 handled Canvas 2D when the compositor returns two mailbox without
prepareMailbox(). This CL applies the same code to WebGL.

BUG=392407

Review URL: https://codereview.chromium.org/252813002

git-svn-id: svn://svn.chromium.org/blink/trunk@177830 bbb929c8-8fbe-4397-9dbb-9b2b20218538